It's not too hard to cut a hardwood block to fit snugly in there and shape the top with a groove for the bolt to clear.
Also know of a homemade sheet metal clip made for that purpose.
Where there's a will
There's a way I suppose.
Sorry, no pics available.
A corresponding groove in the wood.
I have seen one of these before.
The magazine was used for the measurements.
The wood block had a spring clip screwed to the top so you snap the round in and release the bolt and the bolt shoved the round into the chamber.
The clip was shaped like the lips of a magazine so the bolt slid right thru and picked up the round.