alo

DON'T USE A SHIM. You add an additional layer which may increase the likelihood of failure. If the epoxy is thicker, it does not matter. A loose fit actually makes it easer to apply an even coating. You need to prepare surfaces properly.

Use an extra long tube, preferably steel, for your repair. If it is too short, you may have another failure. The longer it is, the stronger it is.

I would put a clamp around this tube, just above the frame. So if the epoxy does fail, it can't slide down.

I would still make the seat adjustable. Use a seat post which fits inside or outside the repair tube. Cut a slit, and use a clamp.

So before beginning, seek out an appropriate tube for the repair, and an appropriate seat post.