I recently got a Marmot Aeros 3P for Christmas, but haven't been able to find anyone currently selling the footprint that goes with it. So how could I go about making one?
From my experience there are two good ways to do this depending on the weather conditions you expect to encounter.
Place waterproof tarp outside tent
Good for rain conditions or where rocks and debris can damage the tent floor.
Place waterproof tarp inside tent
Good for snow conditions
The reason for the difference is that no matter how hard you try, in all but bitter cold conditions you will have some melting of the snow under your tent. This invariably gets onto the tarp surface and makes the tent floor wet, or icy.
I have never purchased an aftermarket tent floor, because they are overpriced and can be easily made following the steps above.
If you are ambitious you can create a custom floor by cutting, and melting the tarp edges. Then re-grommeting the tarp so that it can be independently staked to the ground. This I have found is usually unnecessary for all but the most fickle travelers.
Best of luck out there.
Perhaps this is too naïve a solution, but I would take a tarp, lay the tent out on top of it, trace the outline with a sharpie, and cut on the outline you just drew.
Get your material (tarp, plastic, whatever you choose) out and set your tent up on top of it, with poles.
Draw about 3" around the outside of your tent. Trace circles around where the poles touch.
Cut on the dotted line.
Buy and install grommets on the circles http://www.metalgrommets.com/prod_gromwash.html, so that your poles can drop into them.
Fold over your edges about 1/2", stitch with a sewing machine, and final seal with a permanent seal tape.
(3" is actually a little wide, but when doing this sort of thing you always cut just a bit larger than you need to in case you make a mistake.)
Take a regular rectangular tarp, lay it out such that it is as close to the tent's size as possible.
Then take any amount of extra that is on the UPHILL SIDE, and fold UNDER THE TARP any excess. Any water that comes downhill is then channeled underneath the groundcloth.