Sorry to tell you this, Partner, but your best bet is to rebuild the box. Having gone through this with two sailboats, i found it impossible to do it right by tucking glass in that narrow slot.  Doubtful you can get it clean and dry enough, there's probably some rotten wood, etc., etc.  If it's not too bad, sail it wet theis season and fix it right over the winter.  Good luck.

With my 50 year old 18' wooden X-Boat (Johnson Boat Works, White Bear Lake, MN), I had a leak at the hull/swing keel box. Along both sides of the four foot long box.  Sailed it all summer, then had three of us in the boat, which took the leak below waterline - imagine 8 feet of 1/8 slot, water coming in fast, barely made it to shore.

I sawed off the knees connecting the hull to the box, then lifted the box right up.

A seem of 3m 5200 (??) Marine sealant, put the box back on, screws and no leaks.  Epoxy would be fine too, I imagine.

Two summers, no problems. With I have a boat lift, and it's all oak.  Rain inside keeps the hull wet - I bail after rains.

And, it's been dry storaged for three summers. Maybe next year, after I finish the CLC Shearwater Double.

So maybe you can find the bad seem and let water keep the wood tight.

Good luck!