There are no perfect Faraday cages that are useful. Or, to put it a different way, if it's perfect then you don't want to use it. A perfect cage will prevent everything from going in or out-- including the power you want to use and light (used for seeing if the experiment went well or not).
So, what you want is an imperfect Faraday cage! But here's the problem: is it imperfect in the correct way? A microwave oven only has to work at approximately 2.45 GHz, so it is quite possible that it doesn't work at all at 4 GHz, or 100 MHz. A quick experiment with my cell phone (AT&T 4G) shows 3 out of 5 bars outside the microwave, but inside it also shows 3 bars. We could conclude that at whatever frequency my cell phone is using is not at all shielded by my microwave.
Whatever cage you use or build, you will have to come up with some way to show that it works at the frequencies you care about.
Here are some ideas for making Faraday cages:
A simple metalized mylar anti-static bag. I'm talking about the shiny silver bags-- not the pink, black, or clear+black bags. For small things, these work OK. Not perfect, but not bad either. Usually the seal on the open end is the weakest link. Fold the open end over several times and tape.
Wrap whatever in aluminum foil. As before, take care with the seams. Fold the foil over several times to make a nice, tight seal.
A "Screen Room". Build a room/chamber/box out of a wood frame. Then cover the frame, on both the inside and outside, with brass window screen. A simple staple-gun can attach the screen to the wood. Fold the screen seams together then solder them with a torch. Cover the floor with a solid plate of metal. Good doors are hard, but you could kludge something together. Create panels to run cables (with filters) through the walls of the screen room. Because the screen is on the inside and outside of the wood, you effectively get double-shielding.
Many companies will have a screen room of some sort for preliminary EMI and ESD testing.