Well, my guess for poly-sulfone for the top cap was not that bad: The HP LTZ circuit, and also the LM399, were heated to about 95°C, and therefore, you need a thermally insulating plastic, which can withstand that temperature. Poly sulfone is able to do so, whereas poly-styrene is OK for 70°C maximum, like in 45°C designs. It would be OK also for that amateur purpose, if it's thick enough: The inner layer would deteriorate over time, but the outer side would still isolate enough. If the whole PCB is placed inside a shielding box, the solder side cap can be omitted, of course. I would place a thin sheet of poly styrene underneath the PCB to thermally insulate the circuit. Frank