I'm looking to demonstrate to class of students how microwave absorption works, but I need something relatively common which I can easily make into a board. Does anyone have any suggestions for substances which I could use (currently my best idea is sandwiching a plastic bag full of gel or cheese - something viscous enough not to leak easily, but with a high water content - in between two boards, effectively making an absorbing plane to contain the microwaves) for this?
Oh, and for more information I'm using a microwave transmitter, receiver and oscilloscope for the rest of the setup.
Thank you for your time!