1: Aluminium forms an insulating oxide layer so all your seams probably made poor electrical contact. The resulting slots, if over 6cm long would be very good at transmitting 2.4GHz band RF. To be reasonably effective you cant have any holes greater than 10% of a quarterwave, which is 3mm for the 2.4Ghz band. The cheap DIY option is tin plated steel, with all seams soldered or taped with copper foil with conductive adhesive. For very small enclosures unetched FR4 PCB with all seams soldered is a reasonably good option. You can get a bit more screening with double sided PCB with the edges bevelled, soldered inside and out so its effectively a double Faraday cage, insulated from each other and separated by the PCB thickness. Whatever you do, you'll need a door gasket as well - a copper braid crush gasket is cheap but a total PITA from a maintenance point of view.
2: You can get expanded fine meshes in brass and copper down to about 1mm hole size, and that would be your best option for a viewing port for a DIY Faraday cage, but it doesn't come cheap so you'd probably want to use five solid sides.
3: The door on a microwave is typically a tuned quarterwave choke RF trap designed for 2.45GHz, so it will be less effective at the limits of the 2.4Ghz ISM band. Depending on how sensitive what you are doing is to low level leakage, you may need to clean the paint off and supplement it with a thin highly conductive elastomer crush gasket.