Rubber bung - or other - to turn millstone in to water feature

We have built a little pumped pebble pool in the garden and have put on old mill stone on top. It looks lovely but the hole up the middle of the stone is 95mm and we cant seem to get the water to bubble up gently

Tried building a bung with foam,its better but it isnt sealing the hole and water goes back down the hole - anyone succeeded in getting a millstone to work in this way? My thought was a 100mm rubber bung but no idea where you would find one

Could you fix something in with silicone sealant? My dad built his little pond and water feature himself, and its mainly held together with sealant and stuff from his garage!

thanks feargal - thats sort of what I have done so far, its a mix of stiff foam and a chopped up lemonade bottle - almost works (it leaks rather) but is very Heath Robinson. I could mortar in a pipe but that feels a bit permanent for an old and lovely mill stone

found a sewer pipe tester thing:

https://www.ebay.co.uk/itm/Bailey-BAI2416-2416-Drain-Test-Plug-100mm-4in-Brass-Cap-FREE-POST-/122732674170?_trksid=p2349526.m4383.l4275.c10

Anyone know if that would fit and seal a 95mm diam stone hole?

It may do and seal it with silicone its a try and see thing . Or try cutting a wellington boot up and get a piece off that and silicone that over hole / make sure everything is dry and leave for a few days give it 3inch lap over hole:hihi:

How about a wooden bung fastened in with sealant.I assume it’s 100 mm diameter.If you think it’s feasible I’ve got a wood turning lathe but would need actual required dimensions.

Just to update - we found the Bailey valve thing worked perfectly, even though the inside of the millstone was quite rough. No messing with sealants or anything. Bit of Vaseline on the threads helped tightening it up though

For anyone else thinking of going down the route of turning things like this in to a water feature it works really well. The valve also comes with a standard 1/2 plumbing thread n the middle if you want to attach nozzles etc. And they come in a range of sizes from 2" up to huge, prices vary wildly but we got a 4" one for a tenner