Sigh... I am not questioning your age for the right to play. But I am very strongly suggesting that you cover yourself.
Building lens protectors, making clothing and gear, even making propane adaptors is far from the same laws and regulations that apply to making anything at all that shoots something. Allright?
If you plan to sell them, it becomes a business of selling an item that shoots a projectile. This is where being legally an adult will make a difference. If you were just building one for yourself, probably would not matter. But you did say you wanted to sell them. That changes things.
This can, and will, attract attention to yourself. The income you make from it is part of the issue; ever tried to deal with the Tax Man? It's a lot of fun.
Then when it's notice what your product is, it will attract even more attention.
So I am, again, suggesting to you that you cover ALL the bases if you wish to do this. I am NOT saying you should stop, ok? Just check all your options.