Yeah, it's not a simple answer.
Piezos have a different frequency range and response than your normal guitar pickups... they also have different impedances and output levels.
One of the many, many projects that I have on the backburner is to get a piezo pickup and preamp working for one of my guitars.
So it takes way more than just a capacitor. While guitar pickups have a high output impedance, piezos have an even higher output impedance. Their output levels are lower in general, and they have a narrower and lower frequency response. Because of these factors, you can't just plug a piezo in to a guitar amp and get a great response. You know that, good. So you need a preamp.
You can do the preamp one of two ways - either as an outside pedal or box, or internally in the guitar. It's a bit harder that way, but the sound quality will be better. With high impedance signals, you want them to run as short a length as possible, because they lose tone very quickly. Low impedance signals suffer no such loss - so, we can set up a buffer preamp and achieve at least a signal that will interact well with guitar effects, even if it isn't quite as loud - although some degree of amplification could also be included in the preamp.
You will almost certainly have to design this yourself, so you better start studying. You can do a google search on "piezo buffer schematic", "guitar piezo buffer schematic", and "piezo guitar preamp schematic", etc to get your started along your specific line of questioning. To build this you will need some basic soldering skills, a schematic you've found or designed yourself, components, etc.
I strongly suggest a schematic based around JFETs, as they're both cheap and simple to implement.
Umm... let's see... mimicking the frequency curve of a good pickup is not necessarily that easy of a task. Try googling for "guitar pickup frequency curve", that can give you ideas where to set your -3 db points for the filters you'll need to incorporate into this preamp. I bet you could do a cascode stage followed by a Big Muff tone stack followed by a Fetzer Valve stage. Your total component count should be under 20 pieces. It won't necessarily be easy to skoosh all of that into a board small enough to fit inside a guitar's cavity, but it could be done.
Alternatively you could probably get away with just a Fetzer valve, and do your filtering outside of the guitar. I think you can buy premade PCB boards of many different preamps that will simply give you a boosted, low-impedance signal - that would save you a lot of grief over designing and building one yourself.
Saul