If you've got access to a vise all you have to do is to track down a couple of old steel forks, the crappier the better, and stand them upside down in the vise. At least one longer one makes it even easier. Spread the longest one by standing on one prong and pull at the other. Use this as trueing stand for rear wheels. Run screws through the prongs at the level of the rim to act as adjustable indicators for trueness. A loop of tape or a rubber band around the prongs will give you a roundness indicator. If you have the ability to do some fabrication you can cut off the steerer tube and attach the fork to a thick wooden board (old countertop works well) if you want the ability to use it on a table instead.