If you don't care it being a little flimsy and just want quick, and the plastic is not too brittle:
Use a barbed fitting with a straight thread on the outside, e.g. #10-32 of M5x.8 (these sizes happen to be interchangable), and a sealant.
drill 4.2mm (work up size in small increments to avoid cracking)
tap hole M5x.8 (or #10-32) (with great care, again b/c cracking)
install fitting M5x.8 (or #10-32) x barb
(type intended for 3mm (or 1/8" = 3.2mm) tube)
add silicone sealant (but make sure gasket is not silicone)
(may need to abrade and clean surface to get sealant to stick well)
Illustration of fitting type ... you can probably find something for $1 if you try
If the plastic is brittle, as is pretty common for optically clear materials, a glue-in barb fitting or a bulkhead type retained by a nut on the far side, should be used instead.