You can shield the IR dectector with any Non-transparent/Non-translucent medium, like cardboard or plasti-card.
Hiding the dectector would block sensing hits, not fair. If you hide it behind a semi-transparent medium (like a dark colored celulose sheet), you would still score hits.
You should not need to shield the circuit board, just hide it. Try making up a stack of ammo boxes (Made from balsa wood).

Hope i understood your question and provided a useful answer.

best,

Splat