For uniformity of the gusset you are forming, I would build a simple form right off the start. One piece the size and shape of the gusset, on piece to mount that to, and a female piece to clamp down over it makes a simple effective mold. Glue/screw the first piece on to the solid backer board. then once you have the leather nice and wet place it skin side down on top of the mold - use screws or clamps to hold the female piece down over it and leave it for a day. I've used this setup a couple times for small camera cases etc and it's pretty effective AND cheap ;0) Just make sure all of the wood contacting your leather is sanded smooth so as not too leave unnecessary marks on the hide. And yes, I do this with 8 oz leather. Should give you a nice stiff gusset for the case once yer done, and the lack of flexibility should help keep things in shape while stitching I think.
Lookin forward to seeing it - planning one for myself as well
I can take and post a quick pic of my mold if my description isn't clear - picture worth a thousand blah blah blah ;0)