Hello there, mitch.
Here's an article recently posted to C. B. Gitty's website titled Piezo Pickup Basics: Using Disk Piezos In Cigar Box Guitars - Part 1 that you may find helpful.
For use in your CBG, you may find it helpful to use unencased disk piezos, or at least remove the casing before installing.
There's also, of course, varying opinions on what size disk piezo to use, and where and how to install them. I think you'll find the article linked above provides some insight into all of that.
Personally, I despite some of the drawbacks (extreme sensitivity among them), I really dig piezos, and blabbed about my two cents on them here.
Although there are some extraordinarily talented builders who put a lot of thought into how they use disk piezos in their CBGs, I lean towards hot-gluing a 20mm disk (that's been soldered into a pickup harness with a jack and volume pot) onto the underside of the cigar box lid, right near the bridge.
Since the necks on my CBGs run under the bridge, I glue the piezo to the closer to the side with heavier strings.
I hope some of that is helpful, mitch.
G'day Donkey, good info mate, very helpful! I read your "two cents worth" as well, very informative. I think I may be leaning towards rod piezos now , because I tend to get a bit heavy handed with a slide too.....!