Easy way to gage the relative suitability of a plastic for RF is the microwave test. Place a microwave safe container with one cup of water in the microwave and place your test item in the microwave.
It's not only boxes. I built a resonant transformer arrangement to couple 4MHz ultrasound signals onto my rotating Ph.D. experiment: http://n3ox.net/files/us_ring.jpg The setup is essentially a pair
And folks winding tesla coil secondaries onto ABS DWV (sewer) pipe.. (double whammy there.. carbon black pigment AND the pipe is actually foam with a skin on the inside and outside, so it breaks down