I built one out of an arbor press and it works great for stand-up targets, but ramps can be a struggle. I have a friend that swears by his third hand style made by these guys http://www.arbortime.mountainminded.net/html/the_better_tonka_rivet_tool_-_.html
I bought a couple of edge clamps and plan to convert one using my existing dies. It's like 4th on my todo list