Well, My old pen press was based on one of those linear toggle clamps.
With the pressure used to push together pen parts, it would flex and the front supports would sort of bend. It worked ok, but I thought I could do better. It didn't have all that much travel, so I had to readjust the stop block position pretty frequently.
So after about 1.5 hours and some shop scraps, This is what I came up with.
It's got about 4 inches of travel. The stop block adjusts by rotating it.
Seems to work pretty good...