Note: Pictured is the mintyPi Lite drill guide. v3.5 drill guide looks similar but includes holes for L2/R2, and USB port and headphone jack on the front.
You can use this drill guide to drill holes in your Altoids tin in the correct spots. Just snap it on the bottom, drill pilot holes on the side for the power ports, and drill your L/R buttons (and L2/R2 buttons if you're doing a mintyPi v3.5) on the back using a drill.
Note: this is sold separately from the rest of the mintyPi printed parts because I anticipate most people will want to just get a laser-cut tin from Helder: