I think I understand where he's coming from.
Take a piece of plastic pipe, and cut of a small ring, maybe half inch depth say.
Cut out a disc from acrylic or lexan the same diameter as the pipe, and bond this onto the ring to cap one end off. This effectively makes an end cap. Then using a gasket fit the two ends together. Clearly you will have to think up a method compressing the gasket/seal if using axial methods.
Couple of reasons I would advise against this approach. Firstly the join between the clear plastic disc and tubing is a potential failure point, and you should do as much as you can to eliminate points of failure.
Second, it's unecessary work. All you need is a disc compressing up against the end of the pipe with an o-ring seal. The end of the pipe needs to be well finished, and nice and flat if you're using axial sealing.