[SloopJonB]

A good trick is to make the internal brace as a cross rather than a collar or lip - see attached. This was a keel step but the same concept applies. It keeps water from sitting against the tube and corroding it while still locating the mast properly.

You need to ensure the inside corners are limbered for drainage.

Fit should be snug-ish - maybe 1/32"-1/64".