After a bit of Googling and considering my options, I ended up ordering a .580 push through sizing die from Lodgewood Mfg. Col (http://www.lodgewood.com/Push-Thru-S...Die_p_491.html). It will certainly be better than anything I could fabricate, and isn't much more expensive than the drill bit I would need to buy for a do-it-yourself attempt.

Expect a range report in a few weeks.