It seems like you have two choices - assuming that your coaming has some over lap material (not trimmed to exact length).

1. you could sand the tight spots which will relieve or correct the uneveness of the oval and hopefully allow the loose areas to touch.

2. you could fill the spaces. You could epoxy the coaming in and mix some epoxy putty (colored to match best you can) and push that into the spaces. better darker than lighter. Mask off the upside of the deck and put tape on the underside to capture the putty.

I your spaces arn't too big and noticable, I would go with #2. Once it's cut down and the lip is on, you'll never see it. Your space between the lip and deck should be 3/4".

3. you could also go with thinner coaming laminations (more of them) which would probably conform to the hole cut-out - even without wetting or steaming.

If you planed or sanded your coaming stock thinner it might work.

Or... you could lay your bent coaming on top of the cut-out just over your cutout line , tape it there and trace it on the deck. and re-cut the opening a hair larger to match the coaming curves. the problem then would be to get your second layer to match perfectly. You would have to do a thinner second lamination with more flexability.