Both sides are embeeded in wood, although the wire side appears to be cork on the Amazon.com pictures. I don't think it will matter much which faces the soundboard, but you might experiment both ways. It will be more important that you have the disc tightly and evenly laminated within the wood case, which I suppose is one of the reasons why there's cork on the back side.
Of course, CB Gitty has done a bunch of the trial and error for you already. Might be worth just paying the few extra bucks for the prewired set up with volume control.
There are a bunch of ways that piezos are used. Some just embed them in a gob of hot glue, some potted in bottle caps with silicone, some just foam double side tape to the soundboard (I've done this one). I remember someone just wrapping the disc in tissue or cotton rag and leaving it loose in the box.
if the piezo is sandwiched in wood, it probably doesn't make any difference it's orientation vis-a-vis the soundboard. If it is embedded in wood with only one side exposed, I would definitely orient the the brass toward the wood (away from the soundboard). I've build plenty of "martin style" bridges, with piezo embedded brass side toward the void on the underside of the bridge. before gluing the bridge in place, I cover the ceramic side with silicone caulk. the arrangement cuts out feedback and lessen top noise.
The disco-tone from CB gitty is embedded in a hardwood that looks like oak, but Bad Finger is right that the wire side is cork. From what I can see on the one that I have, there is some sort of silicone or hot glue buffer between the actual piezo and the cork and I would imagine it's is covering the entire piezo. The instructions tell you to put the hardwood surface on the underside of the sound board near the bass side of the bridge (so corkside down). The hardwood has double sided tape already attatched to it to affix to the sound board.
As mentioned below, CB Gitty has probably done a lot of experimenting and the time attempting to replicate this will likely not be worth the extra few dollars.
Piezo elements have a really high impedance, like near 1M ohm. Using a small pot won't work, you need to use one that is at least 250k, if I recall... it's around here somewhere.
Your best bet is to use a buffer/booster and set the volume there. You could install a 9V buffer IN your CBG, if you know a little electronics. Google Jfet preamp.