I don't have experience making a seat bag, nor dry bag harness, but I have experience making stuff sacks, sails, sail covers, etc. so here's some suggestions.  I'm interested what others may say as I'm thinking of making a seat bag.

I suggest making a prototype of cheap, weak fabric (either thin nylon, or even an old bed sheet).  The goals being to get the shape and dimensions right and to test it to failure to see where you need reinforcement or to re position the straps.  Ideally you could disassemble a good prototype and use it for a pattern, or build it from a hand drawn pattern and reuse the pattern on better fabric.

You might even intentionally sew it with weak thread to see where the thread, or seams part first.

I'd love to borrow a 'store bought' bag to make a copy, but unethical to buy it, copy it, then return it.  And you don't want to be attracting any negative karma to the project.  But if your friend has one you could borrow, that might be ok.

It doesn't *seem* that hard.  For a dry bag harness, perhaps using safety pins to temporarily connect intersecting pieces of webbing (not pinned to the dry bag, but pinning straps to straps) to test their position.