If not the best Faraday cage ... not at all, the microwave is a handy size, has a door, hinges, latch.
I've used them in a pinch with these mods:
( remove paint as needed to get a bare metal bond )
1. copper tape over the microwave inlet to the chamber
2. copper tape over the chamber light opening
3. copper tape over the hole where the platter motor spindle "was"
4. copper mesh over the door including the gasket / trap area
5. wire mesh over foam (like Wurth products) gasket, centered in the trap width, do entire perimeter
Depending on how well the inner box seams are sealed (weld, spot weld, crimp), you may have a pretty good chamber at this point.
All that said, cookie tins work great as do larger containers that Christmas popcorn assortments come in. Test for continuity and sand off the finish where necessary in the lid to can area. Don't assume that a shiny gold or silver area is conductive. I've found that frequently they are not.