Best materials to prevent reverse engineering of PCBA components?
One version of the history of modern electrical engineering states that it arose out of the need to systematically study circuits without disassembly, because it had anti-tamper measures inside (explosives) or you didn't want to have evidence of tampering (all during war time). This is were electric network and circuitry theory arises from.

If as you say there are nothing but passives inside your circuit then there is no need to disassemble it to determine what is happening inside. Thus there is no need for tamper prevention.