Congratulations

Take some photos of proper vintage sporter magazines along with that big-box you have to a full-custom chopper build shop (like what you see on the TV shows), have one of their 'tin men' form you a properly shaped bottom plate (can likely cut from a throw-away front fender), cut the box, and silver solder the floor plate in place.

From there you can either choose to have it engraved or just have it rust-blued in the old fashion.

Bang, you have a proper sporter magazine.

Cheers
Tinker