Depends on how big the difference in diameter is as to the preferred solution.
If it's small, just put some brown packing tape on the beam for simple mould release (make sure it's a smooth surface, overlap of the tape is fine though) and squash beam straight onto the mounting with glue between. Pop beam off and clean up the bog (as suggested, colloidal silica or microfibres; not fairing or lightweight filler additives).
If the gap between beam and mount is big, might be best to mould a fibreglass mount around the beam first and then glue that in-place to take some of the bearing stresses out of the cured bog, otherwise it might crack.
A big gap might be larger than 5mm. Any bigger than this and the filler might start to crack when the boat flexes.
Make sure when you redo the mounts that the boat will be aligned correctly once it's bolted up.
Will putting chopped glass in the resin mix make it less likely to crack under stress? Is there a problem with using the chopped glass and 404 filler in the same mix?
The putty rings would work well I think.
I aim to keep it so that the beams can still be removed, although I guess it doesn't matter too much.
