I recently made one for my Hexamid Twin in an effort to save weight over the one I made earlier out of Tyvek. I used an Acco/ACE Hardware outdoor window insulating kit to make mine.
I made mine to be bathtub-like. When set up right, it has a 2" raised edge all around. Of course, the edges tend to flop over, because it's not a perfectly cut bathtub insert. I used gorilla tape to attach little loops of cording to clip into the Hexamid.
I found my biggest problem is that the ground sheet readily slides on the netting. In my first test, I pitched my hexamid in a place with the slightest slant toward the door. I was using it solo this time, and I placed myself at the back of the tent, and bits of gear between me and the door. I woke several times at night to find the whole floor was sliding toward the door, stretching the back side clips to their max.
I'm going to try and address this with some dots of silicone on the bottom of the polycryo insert.
I would be interested to hear more about other experiences with the polycryo under the net rather than over it.
Any worries at all about pitching your tent in a wet area? Of course I try to avoid this at all costs. Thinking back a few years, however, there was one designated site on Isle Royale at North Lake Desor that was horribly wet and muddy after 24 hours of rain. At the time, we were using an REI Quarterdome, and I was very thankful for the bathtub floor. There was no avoiding the mud and runoff without breaking park regulations and camping illegally. I don't know how I would have dealt with those conditions using the Hexamid and a piece of polycryo under the netting.