In my younger days, I used a full-length foam pad -- cut into two pieces:
1. 60" x 20" - main section
2. 12" x 20" - foot section (also seat pad and wind screen)
A brand new piece of blue foam does look unwieldy, but it really isn't. I like to fold them flat and slip them into my backpack -- flat against the pack's back -- then pack everything else in. This makes for an excellent, multi-layer "frame" for my frameless pack -- much, much stiffer (better) than putting the pad in as a circle.
If you like the idea, then do this:
1. Carefully fold the main pad length wise (60" x 10" x 2 layers)
2. Then fold into thirds (20" x 10" x 6 layers).
3. Then have all the corners aligned nicely and weigh the thing down totally flat for 24 hours with heavy books (encyclopedias and dictionaries are perfect).
Now, your foam pad will "remember" this folded position -- making future foldings a snap! To pack, simply fold the main pad as above -- add the foot section -- and slide both into your backpack, flat against the back. Efficient space wise -- and turns any frameless pack into a wonderful load carrier. :)
Finally, feel free to add velcro strips to the two sections for easy connecting at night.