I've dimpled a hole or two in really tight spaces (narrow end of flap/aileron ribs) or middles of panels by taking a small block of steel (or a bucking bar you don't use often) and drilling a shallow hole in it. Then countersink the hole about the same as a dimple. Use this block like the female die, and either (carefully!!!) use the male die plus your rivet gun and a flat set, or put a rivet in the hole, and slowly dimple it with light to moderate controlled hits.
I have been told (with no confirmation) that this is basically how Van did the dimpling on the first RV-3.