I may be making this busier than necessary, but still want to leave a way to get the cushion in and out yet snug up the cover. Here's what it looks like attached to the seat frame (still 'practice' material); also, the fabric top is not glued or otherwise fixed down to the top of the cushion, therefore the rice bag for photogenic purposes.
This is done with the bottom in four sections - a front and back piece that come around the sides, then two overlapping side flaps, and it all velcroes together, plus has the Velcro strip for attaching to the top of the seat frame:
This should work OK, I just need to trim the bottom pieces a little tighter for a better fit. The top material is cut to fit to the edge of the foam, and the seam allowance (I'm using 1/2 inch) compresses the foam for a nicer fit - I didn't do that for the bottom and it shows. Alternatively I could sew the bottom in just two overlapping pieces (a la the rear seats) and count on the existing Velcro on the top of the seat frame to hold everything.
There is also a front and rear strap that wraps around the seat frame and sticks to Velcro on the bottom.
I'm ready to do this as final, and start working on the seat back. Then all that'll be left to do is bind the carpet pieces. Oh, and put the cabin top on and so on and so forth...
==dave==