No, because microwave ovens run at around 2.4GHz only and many things block that frequency. If you want to test the effectiveness at specific frequencies, it is better to put a receiver in the box and see if it can still receive. Although it is a safer method than setting the RF in your microwave oven free, the broadbanded nature of these EMP events makes this type of test kinda pointless.
You could always build an explosively pumped flux compression device and test it yourself... Wikipedia has some really nice drawings on how they work
Obviously I am not suggesting building a big one, but perhaps something the size of a beer can. You should be able to get a couple terawatts out of a device that size. They are disturbingly simple devices to build