jimp, sorry I'd missed your question at the end of your post. When I laid-out the box, it's cut the same radius as the front half of the kayak so though it won't be a "tight" fit, it shouldn't move much.
Got a seat update: After last weeks fishing trip and spending several hours on the water, even though I'd beached and stretched a few times, the following day was a pretty tough one. I have some extenuating physical issues, and couldn't walk the following day. It was nearly impossible for me to exit the kayak at the end of the day. It was bad enough that I knew I couldn't (and honestly didn't want to) use it again without making some changes.
So I followed RetiredRR's lead, and got a $10 plastic patio chair today, took the sawzall to it after some careful eyeballing, and sliced 'er up to fit. Setting in it was a world of difference from the seat I'd made, and even though most people would have found it comfortable, I wasn't one of them. Now I set at a level right at the top of the kayak. I'm curious to see how much difference it will make by setting about 4" higher, but I could already tell it will make entry/exit much easier on the knees and back, and the additional width of the seat and back make a big difference as well.
After an initial run, and confirming it will do what I need it to, I may add a couple rod holders to it, since it (chair) has arms I just as well take advantage of them. I can also add a few small holes in each arm, and hook things such as my cricket cage onto them, and free-up some floor space.