Great that you’re asking. No sense messing up a great board.
Use an epoxy. Resin Research makes great surf resin for this kind of thing but any surfboard shaping supply will have 2 part surfboard epoxy.
Be careful, first don’t ever use polyurethane resins for EPS foam boards. Most SUPs and Kiteboards will use EPS. Poly resin will melt the EPS. Just stick to epoxy, much safer and it works on all kinds of foam safely.
Also, don’t ever pour more than say 1/4" of resin into a space as it heats up when it sets and will melt the EPS. So pour it in stages. Also mixing in Cabosil or glass micro balloons will slow the exothermic reaction and reduce heat.
A lot of folks in board building drill out a larger hole than they need, fill it with epoxy and micro balloons mixed and then drill out a smaller hole so that there’s no exposed foam to collect water. EPS foam sucks water in and is hard to dry so be careful to not let any water into your board or a few weeks in you’ll have a 100 lb board. a lot of surfers weigh their board before they start surfing it and write it down somewhere. That way you can always tell if it’s leaking and taking on water.
Experiment on some packing foam first and get a feel for things before you tear up a good board. Composites are not that hard to work with but they can punish the casual user. Test, play around a bit, and then work on your board. And wear a dust mask at least if you’re drilling or cutting fiberglass. Tiny glass fibers are not good for lungs.
And NO ACETONE to cleanup epoxy. It can make you sensitive and have a reaction so that you can never be around epoxy again. Clean up with white vinegar or denatured alcohol or orange hand cleaner works too.
Good luck, this stuff is truly fun.