well, yea marine epoxy. Some thin sticks to spread it in there, very thin and you will want to get some toothpicks to get some on the edges where it meets with the main piece to keep that waterproofed. best to test on something first. You are going to need to clamp it for awhile after you get the epoxy in there.
1. dry the board and the damaged area. might want to pry that open JUST A TINY BIT and LISTEN CAREFULLY for any CRACKING sounds YIKES! Get that aired out and maybe even a blowdryer on low / medium heat about 6" away for about 5-10 minutes waved back and forth will ensure it is dried out.
2. take a flashlight and look for any junk in there, like sticks, gravel etc. Get a THIN piece of wood that is stiff or metal and clean out any gunk.
3. test run. try carefully closing that flap about 50% of the way to make sure it doesnt get hung up on the sides from the main piece. If it looks and sounds good, try going all the way.
4. if that all works out, get your epoxy ready. make sure you read the instructions carefully and get a LONG cure epoxy. The longer, the stronger, and THE EASIER TO WORK WITH. Ever work with epoxy? It can be challenging, so be sure you know what you are playing with. You dont want it to cure/dry before you finish your repair and end up with a permanent wing on your snowboard!
5. Slather some epoxy in there as far in as you can go with something thin. You'll figure it our, your human. Use something thin, DISPOSABLE, and have several of those same devices ready. Thin piece of wood like a popcicle stick or thinner.
6. Get the toothpicks and dab a very fine line along the edge.
7. Push the flap down, grab a paper town and quickly rub that stuff that squished out along the edges off, and CLAP that bad boy. Make sure the entire repair area is clamped COMPLETELY. If you have a C clamp that only clamps a small area, use to small block of wood to and clamp that.
PUT A PIECE OF WAXPAPER OVER THE REPAIR AREA!!!!
Otherwise, you WILL be prying your clamp material off of your repair job as slight amounts of epoxy will eventually goo out and adhere to the clamp or clamp block! The wax paper allows you to remove those and just have to deal with some wax paper which, should pull right off.
Use whatever means you have available to you to scrape off any excess that ran out and then go riding
I'd leave the clamps on for the FULL amount of time it states to FULL cure before you remove the blocks and go riding on it.
When all is said and done, if done correctly this are of your board will be stronger than the rest
As for that edge up front, you are on your own buddy lol...