Here's roughtly I did:
Materials:
2" PVC coupler (slip)
2 2"-3/4" flush reducing bushings
1' 1/2" PVC
3/4"-1/2" flush bushing
3" 1/2" PETG
Rubber washer seal thing from home depot

Start with a 2" PVC coupler and two flush bushings reducing to 3/4". Make a sort of tank out of this.
Next, sand the rim off of the base of the 3/4"-1/2" bushing so that it can fit into the 3/4" opening the way it's not supposed to (this will allow you to use the stock, clean, unsanded end for barrel attachment).
Run the 1/2" PVC through the 'tank' and into the 3/4"-1/2" bushing so that it is held straight in place. Use some epoxy putty to seal the end without the 3/4"-1/2" bushing.
After it has set (about five minutes) remove the coupler and seal off the inside of the connection with plumbers goop for good measure and let it dry.
Cut down the 1/2" PVC running through the tank so that it comes up about 1/2" further than the wall of the bushing. (You'll just have to play with this to make sure it's the right length by assembling everything and cutting off a littlle bit at a time until it's the right length).
To make the poppet, simply superglue the washer to the PETG. Make sure you only glue around the rim where it's attached. It doesn't have to be super secure because of the nature of the poppet. There's never much stress on the connection because when the poppet is "sucked back" the suction is acting on the rubber washer itself. This action, instead of pulling the PETG away from the washer (which would happen if the whole thing were covered in glue) causes the rubber washer to push the PETG as it gets sucked back, so there's never really a chance for it to come undone.

Stick the poppet into the PVC and do a "blow test." The poppet should easily move forward when you blow into the PVC and slide back in when you suck. I lubricated mine with mineral oil.
Now you need to put a small segment (about 1") of 1/2" PVC into the 3/4"-1/2" bushing so that when the 3/4"-1/2" bushing is inserted into the 2"-3/4" bushing about 1/3" of the 1/2" PVC protrudes down into the tank. This piece of PVC is going to be what the washer seals against.
All you have left to do is put everything together and seal it up. Attach a pump and a valve and maybe you'll have better luck than I did.

To improve the seal I added a 9/16" o-ring to the base of the PVC that seals against the washer, but it didn't seem to make much difference.
What I'm convinced would absolutely work is a "hemi-seal." Instead of using a flat washer, what is needed is soft (low durometer) rounded washer and a rounded opening to seal against. Something like this:

I'm convinced that if the washer (the red part) were made of a soft enough material, it would not be at all difficult to get this to seal perfectly. Does anybody know of any washers/seals that fit this description?
Any questions? Comments? Suggestions?