Sounds like you have wet/soggy/rotten plywood core under the mast step. This is common with deck-stepped small boats when and of the tru-deck hardware isn't bedded properly. Water has gotten in around the fasteners and rotted the plywood core out, so the tension on the rigging is depressing your cabin top.
A metal plate would help distribute the load, but this is just a band-aide on a bigger problem... you're also inviting other corrosion problems using an aluminum plate. You don't need fiberglass skills to repair this problem, mostly just basic carpentry skills and some epoxy.
- The easiest, least time consuming, and cheapest method; take your mast and rigging down... identify the soft areas around the mast step by feel or walking on it (gently) to spot deflection or sounding with a taps from a plastic hammer or other tool. Cover the area with blue painters tape, drill several dozen 1/4" holes is the areas where the core is gone or degraded. Only drill through the top layer of fiberglass, don't drill all the way through! Get a quart of epoxy and some cabisol or colloidal silica. Thicken the epoxy just enough so that is still liquid and flows but not totally runny... Start filling the holes with epoxy. Anywhere you have a void it will suck up a bunch of epoxy. This isn't the best way because there is no telling how good of a 'fill' you will get in a void that could be potentially massive. This method is usually only used where there is limited rot around a confined area, like with my C22 it was just around the mast electrical connector, just a few square inches.
- The 'correct' method would be to carefully cut out the top panel of fiberglass around the entire effected area. Remove the debris of rotted wood and replace it with a panel cut from a sheet of 1/4 to to 1/2 inch marine grade plywood. Glue in the new core first with thickened epoxy, then glue the fiberglass panel back down. There are several examples of this type of work all over this forum and all over the web.
