Polyflex is a fully sintered compoundable ferrite powder designed for maximum heat absorption at 2.45GHz microwave oven RF frequencies; only $17.95 for 25g sample, volume discounts.
A 1mm thick layer of this material mixed with epoxy can attenuate a signal by over 8db at 2.45GHz. Thicker layers or stacked layers can reduce the signal even further. Click on the Specifications tab for a detailed, downloadable pdf file full details.
Download this data sheet on Polyflex fully compoundable sintered ferrite powder for microwave oven RF.
The image below shows a 1mm thick plate made with Polyflex ferrite powder. The plate is roughly 25x40mm in size & is 80% ferrite powder by weight.