Strengthing your boat with epoxy.
Any old mac can be strengthened with the addition of epoxy fillets. These are just an epoxy putty that's troweled into interior or exterior corners, and smoothed with a radius board. I used bits of stiff cardboard cut into a 1" radius, but plastic paddles would work better.
The fillets greatly increase strength, make the corners look better, and help make cleanup easier.
You'll need epoxy, some silica, and some microballoons. The silica adds strength and keeps the mix from sagging, and the microballoons add bulk without and weight. They also allow the cured fillet to be sanded.