Additional design thoughts: Large threads are expensive to machine so I wonder if a simpler closure could be found? Unscrewing a threaded cap is also harder to manage with small, wet, and gloved hands. Since a piston seal O-ring does not require compression by the latch/closure mechanism, very little force is required.
Something as simple as a pin that can be pulled might work. For that matter, so would duct tape. Two small thumb screws or wing nuts perhaps? It doesn't matter much if they are lost in an emergency deployment. Either way, you need some form of texture (knurling) on the cap so you can twist it out. One advantage of a threaded plug is unscrewing it can help open it against a pressure differential -- caused by temperature or altitude changes.
Providing a method for attaching a lanyard is worth considering. As mentioned before, Acrylic (Plexiglas) is far less impact resistant than Polycarbonate (lexan), but has the advantage of chemical bonding/welding. Consider matching a small lip/step to the bottom plug since you are probably turning (on a lathe) it anyway. That adds bonding surface area and aligns the cap to the tubing. The acrylic cement/solvent makes a pretty slippery joint until it sets up.
