i have similar boat,
bass tracker II,
i also have multiple cracks i repaired
there are different epoxies,
some are too stiff when dry and crack also,
for the under side i use a 3m marine epoxy which is rated for below the waterline of plastic hulls,
i apply several thin coats, letting each dry for +24 hrs before the next,
couple things before applying, sand it smooth and to give epoxy good surface to bond to,
if you shoot me a message next time i get the chance(probably 2 weeks or so) i can check exact name of expoxy i use for the underside,
there are many boat epoxies, probably went through 5 or so before found 1 flexible enough not to crack