Enclosed Utility Trailer Project
Being as this is a "home-made" trailer, you are free to do what you want with it and to it. Manufactured trailers would use specific hinges, etc. Hinges: Find hinges that are at least the height (length) you need, even if you have to trim the width. This would be for strength. As far as the cost of welding--this depends on the shops in your area. Sealant: Go to an auto paint supply store, ask for "seam sealer". This is the grey gooey stuff auto builders apply on seams of sheet metal to make them leak-proof. This seam-sealer does not get hard, just tough as it dries. You should be able to find it in quart cans or tubes like caulking, but it's hard to use with a caulking gun.