Actually, I have a set of "dummy loads" for microwave ovens, don't remember
where I picked 'em up at (gov't auction?)... they seem to be silicone
rubber??? impregnated with some kind of conducting material (carbon? Metal
oxide?) You surely must be able to get 'em from a "microwave tech?" You
can even use a thermometer in them, "nuke" for a certain time and arrive at
different measurements--mainly power output....
I am sure some here have seen these things and can describe them much better
than I....
But, the smaller ones work perfect for my "microwave experiments" (ever
chuck a fly in a bottle and see how long it lives in microwave environment?)
(chuck a BIG one of these dummy loads in, and the oven won't even warm a
glass of water, or seemingly, harm a fly!!!)

Regards,
John