How thick of metal for frame connectors

How much water is going to be inside the connectors?

I'd bet you introduce more water to the inside of the connectors drilling drain holes in them and driving in the rain than any condensation which may occur. Better deburr the inner edge too so there is no high spot.

Spray the inside with paint if concerned and move on.

memike, those connectors are on my orange 70 Swinger.