This is a simple but quite useful design for me.

The large slot corresponds to a 29.01mm OAL cartridge.

The small one is for a case that fits within the corresponding 19.01mm size, so it doesn't need to be trimmed.

The holes, according to the photo, are:
C: Thick base (very thick)
O: Exact diameter where the case should sit for proper headspace (if it goes in too far, there might be a problem; if it doesn't fit, something is too thick).
P: Passes through. This is the classic gauge to ensure the measurements are within range and it will fit the chamber without problems.

A little cleaning and deburring is needed to get it right, comparing it to the cartridge you want to use.

A comfortable version has 5 walls, three bottom layers, three top layers, flattened horizontal surfaces, and 35% infill.

Generic Hellbot PLA.

I hope you find it useful and please share or leave a comment.