Suggest you stick a rubber bung in there to stop any damage in the meantime, but for a long term replacement a decent rod builder / repairer should have something lying around to fit and match....
The one I replaced I took the opportunity to get a reel seat with a threaded hole in the middle of the butt cap, which cheerfully took a 9" home made screw in fighting butt (a length of stainless steel tube with a male threaded ferrule araldited into it and cork rings on the outside then sand to shape.... job done !). When not using the long butt I screwed in a shortie mushroom head type bit of cork.
Not an original idea, pinched it off of a Fulling Mill rod.....
In passing does anyone else own one of their rods and find it difficult to cast with ? Seems very very stiff (the 10 footer). Possibly not the rod's fault, just that it is beyond my arthritic and limited casting abilities....