I take it by sleeve you mean an inside sleeve or plug.

I have used:

Wooden Dowel, This is the weakest

Bunch of little fiberglass rods. Put as many little rod inside as possible and bond the whole thing together with gorilla glue. It lasted a couple of years

Hobie fiberglass rudder pin wrapped with fiberglass and epoxy until it just fit. Worked well, the stick broke at a different spot next time.

Made a solid Carbon Epoxy plug (see original post) It has not broken

Build your own sleeve with a slit in it. I have heard of this but have not tried it. Find something 1-2mm larger than the inside with 2 layers of wax paper or release film wrapped around it. Layup enough carbon epoxy or glass epoxy to get the same thickness as your stick. You don't want the edges of the layup touching. You want just enough space to get a screwdriver in, so you can pop it off. Use the electrical tape or peel ply method to compress it. Then pop it off and trim the slit until you can just force it in. Let me know how this works.

Carbon rod or tube or fiberglass rod or tube you bought off the internet.

Tubular or rod batten from a sailmaker. The houston sailmakers have a stock of leftover battens and junk fiberglass and carbon. I can frequently find something that will work. They are nice if you bring a repair job when you go in.