how to prevent IR signal to go through 3d printed plastic?
You can not use common modulation remote control receivers as they contain an ACG feature which will likely "turn up the volume" until it detects even a weak reflected signal.  Instead consider if this reflective sensor, which does not contain an AGC feature, is appropriate for your design.

Alternatively you might explore the response time of the AGC feature to see if you can defeat its intended purpose.  That is to say, rapidly switch between IR transmitters in order to keep the AGC level at low gain.  Essentially blinding it from weak reflected signals.

You might also transmit continuously on all IR sources and determine if the AGC will put the weak signals below the noise floor.