I thought I would share a successful Multi-Use MYOG project I did some time ago...
I started out with a Nunatak Lunapad, cut it in half to approximately 20 x 40, then cut slits about 2/3 of the way through, and reinforced the slits with small strips of duct tape. This allows it to fold into four equal sections and fit into the sleeves next to my back in my GG Mariposa.
I have been using this as my main sleeping pad for about a year now, and find it to be very comfortable. It weighs in at 5.7oz, and because it is so stiff, I was able to remove my pack stays.
Here are a few photos;
Inserted into the backpack
Detail of fold