First, I would talk to my folks about this. They may not be keen on you drilling lag bolt holes through the closet floors.
Second, a burglar will tear loose almost anything not bolted through the floor joists with 1/2" bolts. A small safe just screwed to the floor is a simple task for a crowbar to deal with.
Third, a safe box like you describe may not be the best place to store knives. The environment may lead to handle problems, and possibly rusted blades.
Lastly, storing your knives in a knife roll is the best place for them. You can make one from almost any type of heavy cloth, or buy one from a sheath supply. It is just like the roll tools like wrenches and chisels are stored in. Heck, you might find some old rolls sitting in the dollar bin at a pawn shop or thrift store.
Put this roll in a hard to find place where the burglar won't waste time looking. Don't tell your friends where you keep it, or it will probably be a waste of time hiding it. Most robberies are done by someone who knows you. A fold over type roll is barely an inch thick. It could be placed between the mattress and box spring.
Extra last comment - You are 15, that is not too young to work. You can't get a job at the local factory, but millions of 15 year old boys do odd jobs for neighbors, gardening chores, mow lawns, and shovel snow ( You ARE in MI, you know). Then there are things like raking leaves, splitting and/or stacking firewood, etc. It won't hurt to ask around, and you might even get a job at a local small store cleaning up the parking lot or taking out the trash.
Obviously, you need to ask your parents before doing any of these things. They are on board with your knife, gun, and knifemaking interests.....aren't they?