Andrew,
You are onto the right path on this topic, getting 100dB of attenuation on a table top at RF frequencies is sometimes pretty difficult. We use table top miniature screen room from Lindgren, but if they are not available they I use separation (distance). So what I do is that I use a multiple of 6 foot SMA coaxial cable and 20dB pad on the end of each end.
So to get 100dB, I would have 5 sections of 6 foot coax cable and 20dB pad. Be aware you cannot coil this setup up, you need 20-30 feet of separation between the TX and RX. This setup typically ends up giving me 105dB of loss.
The next issue is if you do not have a screen room, you need to make sure that the band you are operating at is free of "other RF sources", or else the inverse leakage will effect the perceived sensitivity.
Regards,
/TA