In the field, I've used coffee filters, instant coffee and tea bags filled with coffee. I initially built this as kind of a lark and to see if I could do it. Well, it worked so well, I've taken on a few trips.
I built it very much like a soda can stove. It takes three can bottoms; two fit together to hold the coffee and one as the stand at the bottom. The middle shaft or percolation pipe is a piece of an aluminum wind chime.
In constructing it, these are the points I tried to keep in mind:
1) The bottom end of the pipe has to stay off the bottom of the pot (so water can percolate up through it).
2) The the holes in the very top portion of the coffee holder are smaller than the ones in the bottom of it so that all water that percolates in can flow through. Preventing it from backing up. I made these holes with tiny tack nails and a hammer.

If any of the holes for the pipe to go through the middle are too big, cut a small strip of aluminum and wrap it around the pipe. It should snug right up. It needs to be pretty tight since the aluminum will expand when heated and you don't want the top portion sliding down onto the base.

It holds about two tablespoons of ground coffee and letting it percolate for about 4 minutes makes 16oz of fairly strong coffee. Just experiment a few times and you'll find your sweet spot.