I've been using one of these on the diesels when I don't have the sleeve type. I have a 4-6" one too.

Bobstools - Lisle Ring Compressor - 7" - 10" (height 7")

The only things that I don't like about it are: It's sharp on the edges so handle it carefully. And the biggest, make sure the ends are flush. The outer liner will want to slip down and this can allow a ring to pop out and cause you issues. I've broken a ring not paying attention. I'm more aware now. Haha