A stripper clip would work but you would only be able to load four of the five cartridges in the rifle.

Mosin-Nagants were made to take five rounds in the magazine with the bolt open. Closing the bolt chambers the top round and results in 4 down + 1 in the chamber. That makes sense for a military rifle. The soldier charges it with five rounds, closes the bolt, and it's ready to shoot.

The magazine alteration for the fatter .405 Grenadier cartridge results in the converted rifle holding 4 rounds with the bolt open and 3 down + 1 in the chamber with the bolt closed.

That is why you see 4 rounds in the video I posted the link to above.

A master of metallurgy could redesign, cut, weld, and form the bottom of the magazine into something bigger but the effort would be a lot of trouble to go to just to add another round to a sporting rifle.