@Charles Taylor good question, and one that comes up a lot with field deployments.
The good news is ABS is pretty RF-transparent at 2.4GHz, so 3mm of wall thickness is not going to wreck you. You're probably looking at somewhere in the 1-3dB insertion loss range for the plastic itself, which in practice means maybe 20-40% range reduction compared to open air — not half your range, not catastrophic. The bigger killers are usually metal fasteners, any foil gaskets, or moisture ingress seals with conductive properties running near the antenna, so watch for those more than the ABS itself.
The thing that matters most is where the antenna sits relative to the enclosure walls. If the module's onboard antenna is pressed up against a wall or sitting directly on a ground plane extension from the enclosure, that's where you see nasty pattern distortion and real range loss. Centering the antenna in the enclosure and keeping it a few millimeters from any surface helps a lot. If your form factor allows it, orienting the antenna element toward the wall that faces the direction of communication also helps since you're putting the beam through the thinnest path.
If you want to be more aggressive about preserving range, a short pigtail and an external antenna stub through the enclosure wall (just a small SMA bulkhead) is the cleanest solution and gets you back to essentially open-air performance. Worth considering if this deployment is range-critical.
