The upside-down cheap food-box is made of Polypropylene PP5 and after being microwaved with >500W RF it stayed cool! (however, it has no microwave safe mark. This plastic is very sturdy, so sawing-drilling is easy. Only made RL-measurements with the "boxed" Helical feed in the feedpoint of a dish, measured abt 25dB at 2400MHz.
Performance with my current 1.1m offset dish and abt. 3W at the Helix is very good, but could still be interesting to measure for any reduction on 2.4GHz and also for 10.5GHz using this type of weather-protection. I will come back on this.
73 Ed PA1EW
Thanks for the information Ed.
As reported above I have taken your advice and purchased a PP5 box and hopefully can test it soon.
73 Mike G4CDF