Here are a couple on the web that I have looked at. One of them is pretty elaborate for a corner of a room.
If you have a skill saw, a few 2x4s, and some plywood you can whip one out pretty quickly. Once you mount your press, you may have to trim the 2x4 frame so it makes a complete stroke. I had to with my rock chucker.
http://www.ray-vin.com/tech/corner-bench-plans.pdf
http://www.contrib.andrew.cmu.edu/~sodapop/recguns.html
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