Here are a couple of the framebags I've made in the past year.  Both bags are made entirely out of recycled material: old Therma-rest sleeping pad material salvaged from a local outdoor shop that was tossing them out, zippers taken from old raingear that had seen better days.  I'm not sure what the material used in sleeping pads actually is but I can say that the orange bag has about 800kms on it with no wear issues at all and the grey one is in constant use with the bike being the daily commuter.  I've been caught in nasty rainstorms and the bags haven't been penetrated by water.  Lucky considering I didn't do anything to waterproof them.

Orange one is on a L Santa Cruz Highball, The seatbag is a holster I made for a modified drybag and it works pretty well at holding a crap ton of food.

The grey is on an XL Long Haul Trucker.  The bag has upper/lower sections separated entirely with a map pocket on the left hand side.  For reference, the shoe in the pic is a size 11, that bag is gargantuan.

My sewing technique needs some work or, as likely, I need a better machine than the 40 year old one I'm using. 

Used one last year on a 3 day 90 mile tour that was about 50/50 single track and fire road. Rated weight for side mount panniers was 50lbs total, I had 36lbs total, evenly distributed. I did have the side brackets for the panniers, they weren't just hanging off the top rail.

By the end of the trip I had the rack mount system tied together with more bits of string and wire than anything else.   The single track sections just beat the snot out of it.  The plastic pieces couldn't handle the side to side forces.

Had the side panels been anchored to the rack frame, it might have minimized the amount of sway/torsion and saved things.   In hindsight, I woukd have tied them to the frame so they didn't sway.  Additionally, with the shorter chain stays on the bike I had then I found that the rack sat awful close and I did heel strike the panniers a few times until I got them placed comfortably.

At the end of the day, these would have been great if I weren't on rougher singletrack. For fire roads or smoother trails I think these would have been fantastic. I did switch to a seatpack but that just my decision. Good luck woth yours.