your making it too complicated. it’s actually very simple.
What you need is your Receiver, 4″ of liner, and the Fleshlight case with no end cap. For the standard fleshlight, I use a 2″ receiver, and 4″ of 2″ liner. Place the liner over the end of the receiver, then slide the fleshlight into the liner so that the liner covers the ends of the receiver and the fleshlight case. This seals the air gap between the two.
For the Sex-in-a-can series, use a 2.5″ receiver and liner.
you do not get as long of a stroke as the receiver will give you by itself, but it works well enough.