I did something similar, but rigged it up so it had a top punch too.
Get one of the old watch staking tool sets, and modify the bottom to
hold an overgrown beading tool, as per your design, then use the
overarm to guide another one in from the top.
There are enough old staking tool sets floating around out there
that they’re not too expensive. So long as you don’t frankenstein a
good one, there’s no karmic debt to it.