Used to be that guys would have a round metal rail of about 3/4” diameter attached to the bottom of the larger stocks for the rear bag to ride on.
Quite often they were made of brass, but any metal would no doubt work.
I suppose you could even use a wood dowel to make one.
I also did what Fui suggested and reshaped them on the bottom to more of a sporter stock configuration.
I also use just a flat bag on some, as we use a quick responce type front rest for hunting, and all the adjusting is done there.