Easily any of the rimed rounds. 30-40 K, 303 Brit, 7.62 X54R All are just different versions of what the Siamese is chambered in and ammo is cheap and eazy to obtain or load for.
348 Win
405 Win
416 Barns (45-70 necked to .416)
444 Marlin
45-70.
Having done one in 45- 70 and having to file on the feed ramp to get the big flat rounds to feed reliably the 348 with the similar shape as the Siamese 8X50R or 52R it would make it a little easier.
The OAL of the 45-70 is a little short for the mag and any of the necked rounds will give you a long heavy bullet.
I built mine as a Camp cannon when in bear country but the 348 Win would work for that or as a Moose mortor woods elk, or any other animal.