Not being the Martha Stewart type, I've been experimenting with making an spe with little or no sewing. I've posted and seen posts from others on various ways of doing that, and this is yet another one, although it would involve some un-sewing but it should be real easy.
A neat sheet (the small one, 57 x 77) has baffles running length wise that are 8 inches apart (the end baffles are 8 1/2 inches apart to account for the extra inch in width.)
Remove the edge stitching from all but the two outside baffles. You have to remove a few inches of the edge stitching anyway to get out the round corner weights. this will give you access to each baffle chamber.
Then cut up three 20" Wal-Mart pads ($6 each), or two 24" Wal-Mart pads ($12 each) into five 8" strips and slide them into each of the five inside baffles on the neat sheet.
This would give you a 40 inch pad that should wrap around your body easier than a solid 40 inch because it is divided into 8 inch sections. But before you go cutting up pads, we should consider some downsides.
1. Like any baffles that are straight through, you are not going to have much insulation at the seams, and this may create unacceptable cold spots.
2. Each 8 inch baffle may rollup and twist and turn individually to the point where together they are worst than one 40 inch pad.
3. Sleeping on 8 inch strips may not be comfortable.
Alternatively, you could remove the stitching for the two middle baffles, which would combine the three middle baffles into one 24" baffle. Then you could slide a 24" Wal-Mart pad into that baffle and put 8 inch pieces into the adjoining baffles.
If you don't want a full length baffle then you simply hand stitch the baffles shut at whatever length you want.