I agree that Plinker's jig is worth it's weight in gold. I've completed several nice builds with it and am still amazed at how well thought out it is. What's even more valuable is all the instructions and photos that Plinker furnishes on a CD. I printed all this and made a binder that I refer to. Also, Plinker himself has alway been willing to answer my emails when I have had questions. It's really a bargain if you look at all the stuff it comes with and what it does.
I ran into the same problem with rear rivets. I agree with Eyegun, that the lenth of rivet protusion is the most important factor in producing a nicely formed head.
I recently bought a zillion rivets through one of the recent group rivet buys on Gunsnet, so I felt I could spare a few for experimentation. I drilled some holes in a steel bar then cut back some long rivets to simulate the length of the rivet protruding from the side of my receiver. I used Plinker's jig with the cupped set screw. Just the way they came out of the bag, the longer rivets protruded about 6/16" through my receiver. This seemed way too long, to my eye. I started with this length first, which just resulted in me bending the rivet over. I kept shortening subsequent rivets by about 1/16 at time, until I got down to a 3/16" protrusion, which seemed to form a nice, even shaped head. Encouraged by this result, I went ahead and did the actual riveting of my first rear trunion, using rivets cut down to a 3/16" protrusion. The heads were as nice as any AK factory job I've ever seen. Note, after cutting back the rivet shanks to 3/16" protrusion, take care to file the shank surface parallel to the side of the receiver, remove any burrs and smooth up the surface to remove any saw or file marks. When you form the head, any imperfections in shank will be mirrored in the finished head.
Plinker's jig works "ok" for the straight through rear trunion rivets, but even with the proper protrusion, you've really got take pains to eyeball from a couple of angles and make sure everything is lined up straight and is stabilized with the clamp, so the rivet is crushed straight.
Plinker also has another fixture he developed just for the straight through trunion rivets and cross member rivet. By consulting with Plinker and looking at some of his photo's, I made my own copy of it. I've spent alot of time experimenting with it, too. Everything I said above about rivet length and prep etc. also holds true equally ture with this jig, except the setup is fool proof and takes almost no time at all.