I guess I feel encouraged to proceed. So, I've ordered the materials from GG and Thru-hiker. I'll sew the Momentum top, which will include a Nanoseeum bug net (similar to a Ti Goat Ptarmigan) and a side zip, then when that's finished, I'll either sew (if it seems possible) or bond a piece of polychro 24" by 78" to be the bottom. I'll just do one or two lines of stitching so that it will be easy to replace. If it doesn't work out, I'll switch to silnylon or a GG spinaker groundsheet or cuben for the bottom. The polychro is definitely the cheapest UL option, though cuben would be nice. I'm hoping the finished product will be 5 oz or less, which would mean I'm only adding 3.5 oz to my kit (since I use a medium polychro groundsheet at 1.5 oz already.
If anyone has a piece or pieces of cuben that would make a 24" by 78" bottom out of 1 to 3 pieces, let me know what you would want for it.