How about a CZ550 458WIN or 458 reamed out to a Lott? Mine fits and feeds 6 rounds from the mag ,not 2 or 3. The 458 Lott uses the same diameter case as the 375 but smaller than the rigby ,so more rounds fit in the mag. You can buy 375 basic brass from White horse arms in VIC to make the cases, just run them through a 458 Lott sizing die ,maybe trim a bit . If that is more than you were after ,the 458 Win is a factory round.
As everyone says though, if you are new to big bores or recoil sensitive. The 375 is the great alrounder.