For a Great Plains style rifle the round patch box would be the most appropriate. Go with iron to be more consistent with the existing hardware.
Figure out the centerline on the stock. The centerline of the patchbox will be placed over it. Keeping the box in place, scribe the outline of the patchbox. Inlet the box and then drill out the hole afterwards.
Vigilantibus et non dormientibus jura subveniunt. Molon Labe!