Do you have a mouse sander? If you've used too much filler for the outer layer don't panic!
I did that on my first attempt at this sort of thing. Just sand it right back, you'll start to see the card underneath and that's when you stop. Try to follow the card underneath to sand it to the shape it should be. Fortunately Boushh is a pretty simple lid to sand back and you should be fine.
If you do decide to start over though, you just need to skim the surface with the filler. When you're sanding, card will start to show through and again stop there. Then build up a second layer to smooth it further, filling any dips or recesses from the first layer. That's how I did my custom Mando lid and it worked great! Too little is easier to work with than too much.
As for the air bubbles. That's an easy fix. Do you have a Dremmel or similar mini tool? Use the sanding bit, it looks like a drum/cylinder. Use that to sand through the fibreglass and to break through the bubble. Sand it smooth so that there's a patch of card that is no longer covered with fibreglass. Than once you've got rid of all the bubbles, re-fibreglass those areas, taking more time and care to avoid new bubbles. I also use generous ammounts of resin in these tight awkward corners so that the excess resin kinda forms into pools and fills up those recess areas. It prevents bubbles should the fibreglass itself not stay in position(which can happen if you've not trimmed the fibreglass into reqired shape. Remember, the inside of you lid doesn't HAVE to be pretty. So long as it's doing it's job. Unless you plan to decorate the inside too. In that case you need to take care fibreglassing to get the best finish. Cutting fibreglass to shape to fit properly into tight areas. I also use wodden cocktail sticks ur wodden BBQ skewers to press the FG into position in those pesky tight areas.
Small air bubbles aren't the end of the world but the big ones can cause problems.
Looking forward to the pics mate!!