Here's my own slight variant of the Fatty Piston that I saw on Cowgirl's blog. You veterans will be quite familiar with all of this, so please bear with me. I am including it for other newbs like myself and also for general comment if anyone feels so inclined.
Anyway... it starts with the following components all schedule 40 PVC (whatever that means):
On top is a 10" length of 2 inch PVC (cut from an original 2 foot length which is one of the ways Lowes sells it) I actually cut two molds from that length.
Immediately below that is a 12 inch length of half inch PVC (had to buy a 5 foot length of that, but it was like $1.50)
Below that are two half inch end caps
Below those from left to right are two 2" knockout plugs and a 1.5" end plug (I forget the exact name)
Assembled it looks like this:
The knockout plug with the flange on it is used to cap the bottom of the 2 inch mold - fits well but is not air or water tight. The end caps go on the 1/2" pipe, one end to push food down, the other end so it is more comfortable in your hand). I chose this size for the "tamper" because it was much narrower than the inside of the mold... this was important in my mind to avoid air bubbles in the filling... air bubbles are bad news for this and if you tamped it down with a full width tamper, the air might not be able to readily escape if it had been accidentally introduced into the column of food in the mold...
I took one of the knockout plugs and used a Dremel to take off the flange part so it looks like this and slips quite snugly over the end of the 1.5" end plug (you could use a file or sandpaper if you have no Dremel):
That makes a very close fitting "piston" for the mold... and will let you push food out of it and not leave anything but a little liquid along the inner walls of the mold.
All parts "friction fit" together without any glue... this was important to me for several reasons, first, no PVC glue and associated fumes to worry about, second, no concerns about things being sealed or glue melting in a dishwasher, and thirdly and perhaps most importantly, for sanitary reasons... it can be completely disassembled and thoroughly cleaned in a dishwasher or by hand with each use. If you want to clean it by hand, you can use a bottle brush of the proper diameter or just use the half inch rod to shove a cleaning sponge or cloth down the mold... these parts all came out of the dishwasher clean as a whistle for me after shoving my dish sponge through the 2" tube a couple times. I did buy a few extra knockout plugs in case I screwed one up or in case fitting it over the end plug causes it to crack and break over time (which you can see it has started to do in the one picture above if you look closely)... so I have backups.
I don't know about BPA with this stuff or anything like that... the food doesn't go into the PVC hot and doesn't sit in it for very long, and I made sure that everything was thoroughly cleaned before I used it for the food.
I am sure that there would be a suitable device of this nature available from kitchen supply stores, but I found the idea for this pretty handily and it took me 15 minutes in Lowes to gather the parts and another 15 minutes to assemble it with a hacksaw to cut the tubes to length, and a Dremel tool to clean up the ends of all the cuts and to cut the flange off the one knockout plug.