You can sand the edge until you see two separate pieces, then you can pry the lid off. Alternately, use a side-cutting can opener.

This is for the guitar jack. Note: you don't have to put it in the bottom if you don't want. Also note: a conical bit works wonders for this as opposed to an increasing series of drill bits.

Use stranded wire about 2" longer than the height of the can.

If the underside is white, go ahead and sand off this coating first. Don't try to put glue between the piezo and the lid. Just put it flat and glue around the perimeter to stick it down. It's also a good idea to glob some on to your solder connections.

You might need to employ someone with small hands.

I used green foam from Joann, which is basically like that stuff you use around a window unit air conditioner to help seal in the cold. The point here is to push against the lid to reduce feedback. Too much pressure and it will sound woofy. You will want a piece of foam about 2" taller than the can.

Here's where the shirt and screen come in. Cut a circle of t-shirt a bit larger than the lid (I use the ring to trace). Cut down your screen patch to match. This here is the tricky bit -- trying to friction fit it all together without wrinkling the t-shirt too much.

See how your foam is doing. Add a small piece if you need more pressure, or chop it down a bit.

Drill four 1/16" holes around the lid and insert screws. I used 4x3/8 sheet metal screws.