I use a hard plastic tube.
External diameter is near 7mm (or near 5mm for small caps), slightly less than cartridge interior diameter (caps are slightly wider to make good seal when inserted). If diameter is too small, tool could slip into charge. Walls of tube have to be thick enough to make it stiff for pushing.
Over the time I found that loading tools get damaged from hard edges of cartridge. Might need replacement sometime, though mines still work after hundreds and hundreds of caps.