In my study dealing with effect of microwaves on nematodes, I used neoprene container which was most transparent to this radio frequency. I did try pvc and fiber glass which absorbed the radiation and raised their temperature.

You don't want absorbers, you want reflectors, i.e. good conductors, so the microwaves keep coming back as they do in a microwave oven.  If you have absorbers they will get hot as well as, or instead of, the thing you want to pyrolyze.  Having absorbers in a microwave heater is a bit like putting the pendulum of a clock in a jar of treacle, or dragging a lead weight behind your bike.

That is true that reflectors are needed.  Neoprene does not absorb microwaves and would work as a good reflector.

Neoprene rubber is largely transparent to microwaves and is a bit lossy, so will let most of the microwaves out as well as getting hot, or perhaps only warm.  You need to use metal walls, not dielectric.  If you want a container in the middle that is transparent to microwaves, you need to use a dielectric with a low loss tangent, although you might get away with neoprene.  If it is contact with the paper etc. it will also need to be able to cope with getting hot.  You will still need walls somewhere that will keep the microwave power in.  You are not allowed to let that kind of power spray everywhere, and it is very inefficient, i.e. expensive, to waste the energy that isn't used first time.  The kind of power that you will need to use will make absorbers you use hot, which is probably a bad idea.

The following is from

http://www.rfcafe.com/references/electrical/dielectric-constants-strengths.htm

Neoprene rubber

dielectric constant

6.26 @ 1 MHz

4 @ 3 GHz

 loss tangent

0.038 @ 1 MHz

0.034 @ 3 GHz