After much messing about making things overly complicated, I figured out a simple useable design that works.
I just need to work on a better way to secure the bag to the post in order to stop the slight bit of moving around
that still happens. My goal is to have it SECURE so there is no movement in the roughest trailriding conditions.

Like my Handlebar harness this predominantly made out of stuff I had lying around at home and work.
The harness body is from the same 1/16" pvc sheet I found at work. I also made a bracket out of a rectangle of
1/8" aluminum from somewhere A hole was drilled in this aluminum and after some trimming and bending, the bracket was placed under the seat across the seat rails.

A hole in the pvc (I put a banner grommet in the hole because I can) and large washer on the underside of the pvc harness, along with the right sized nut and bolt (bike parts) holds the whole thing tight and stable.

I use a 2" wide velcro strap bought from Home depot. It goes through the rails and around the harness. This takes
the hanging/flopping forces away the plastic nut bolt area as the seat rails and strap. As it is, it is actually more stable than the store bought way smaller bike bag attached to my road bike.
I Just have to work out the post/bag connection and I'm off to my test track for some R&D pummeling:)

I am assuming the open area rolls up toothpaste style to adjust for different load sizes. Could you do me a favour and take a picture of the bag rolled and clipped into it's smallest setting please?

Also, How is it with side to side play? say stand up pedaling /bike rocking for example.

I'm interested to know how this back works out. Does it flop side to side? I read that it doesn t hold weight well AT ALL and flops down onto the rear wheel like a limp ----.

What length is the finished pipe bracket you made please?

Nice! What diameter pvc pipe did you use and what size dry bag please?

and

Now the finish product.

Once I was happy with the strap details, I measured everything out and cut the slots properly
using a drill on the corners prevents the plastic from tearing.

I used dental floss for thread and I think put the clips the opposite way around than normal with the intention of
using tension to close them closed.

The velcro is a backup in case the clips slip and also keps the straps from flopping about.

Nice! I'd be interested in some detail shots of the handlebar harness when you have the time.  

You asked for it!

So first I scoured this site as well as rest of the internet for ideas and settled on a design that involves strapes being webbed through a harness and around the handlebars as there was now sewing. This way I could use the materials I have lying around the shop.

First step was to make sure it would work. I made a basic mock up out of cardboard and simulated bars and a bag with materials I had close at hand.

*EDIT*: Forgot to post a picture of the first lifesize mock up which I used to map out the strap hole spacings and general shape of the working mock up. It's below the black plastic picture and out of sequence:(

Once I was convinced my ideas would work I cut up another rough mock up with the actual material I would be using and experimented with scrap stapping of various widths and wrap configurations. I used this mock up to strap to my bars and try different stabalizing methods. The cuts in the centre were for running a strap through and around my headtube. I ended up running the straps under my forks and up to the front which works amazingly. The center cut holes were still usefull as I use them for the 2" hoizontal strap which is I suspect more of a safety feature than anything else . It keeps the bag inside the harness from moving side to side and helps take off some stress from the buckles. It may be overkill but could come in handy in ways I haven't thought of yet so it stays.

I managed to build myself a pretty awesome handlebar bag harness out of some scrap plastic from work, a few clips, 1" strap and a bit of velcro. I Also scored a small frame bag on clearance from M.E.C. , (the REI of Canada) that barely fits into my weird shaped bike triangle.

 I don't have a compression sack or anything like that so I just put the hammock bag in the harness for now. In the frame bag I just managed to fit my pop can stove and pot kit.
I also had in my backpack an Aeropress coffee maker, a new purchase I wanted to try out, out of doors.

I loaded the bike with gear and headed out for a day of trail riding in one of the roughest trail systems in Ontario. I figured being pummeled by constant rock riding would be a good test for the straps on the harness.The harness holding the 4lb hammock kit worked perfectly. No movement at all. I could even jump with it with no movement or clip failure. I imagine the shape would be more rounded if I used a cylindrical compression sack. I'm fine with how it is though.

Werkinit, you used to ride trials didn't you? Is your name Darren? I used to ride with you guys and Yuri. shoot me an email at iohan.gue@gmail.com ! (starts with an i, not an L - when its bolded its hard to see )

That is me:)

Still Talk to Yuri. He works at Bikezone if you want to say hi.
I'll be at Joyride150 Saturday 10:00am if you want to meet.

I'll contact you via email.

New to this forum and just starting to get interested in this bikepacking thing.
Currently collecting extended hike/biking gear and want to try some sort of ride
this coming summer. Would rather do mtb. I live in currently live in Toronto but have
connections to Guelph, Milton etc.
 I have been looking into the Ganaraska forest as it's extensiveand somewhat
close. Any bike specific info would be great.