In increasing levels of permanence:
1) shim the pylons/casings with strips of brass or aluminum. Take the boat apart, bend the strips in the shape of an upside down "J" and hang the strips on the edge of the pylon, then reassemble.
2) Shim / glue - in addition to the above, remove the castings from the crossbars, epoxy them back on and re-rivet them.
3) Glue the boat together in addition to gluing the castings to the sidebars, epoxy the castings to the pylons.
For #3, you need to have the boat on a level surface and make sure the boat is square while the glue sets.
In all cases, you need a really
tight trampoline. The easiest way to do that is to use your mainsheet system to pull the sidebars inward, lace the trampoline and pull it tight, then let the mainsheet off.