DiBond + 91 deg bit + SB.

Corners can be sealed and adhered with Sika or other elastomeric adhesive - even riveted or screwed. Set it in a jig, let it cure & Bob's yer uncle.

If you don't want the expense of DiBond, then Alcoa roll stock for flashing/trim + vacuum and down cutting O-flute will allow you to create tabs/flaps, 'drill' 1/8" holes and then pop rivet the whole deal together.

Sorry that pees on the idea of buying new tools...but, it can be done with what you already have at hand.

-B