Lynne and I went to HD and looked around for sonotube, but the largest diameter they had was 12". I considered using a smaller diameter and a box, but the box would be a lot of complex wood work with no guarantee of quality. So I decided to build a sealed enclosure. A speaker enclosure needs to have a tremendous modulus of elasticity to minimise sound transfer through the walls, be sonically 'dead' (not tend to ring in the frequencies the speaker emits) and be as dense as practical (to maximise the fresnel reflection due to the maximum difference between refractive indices). Steel tends to have lots of resonances, mdf is good but tends to only come in flat, plywood is reasonable, but is notorious for voids and vibrations.
What about ceramics? I'm not talking space age stuff, but rather terracotta or other similar baked clays. Turns out that they are nearly ideal (hence being popular for drums), if a little heavy.
So we headed over to the garden section and sure enough there were plenty of potential candidates in the $30-$60 range. I wasn't entirely sure of the inside diameter of driver, and wanted to think about the idea for a little bit. A classic bit of Frugology if ever there was one :)
Two days later (after our trip to the SF Garden show) and after our trip to the market we returned to HD with exact measurements.
There were a few pots in the right range (16.3 inches was the inner mounting dia, I was happy to go up to 16.7" and use some weather strip in the case that that wasn't an option), but they were either expensive or cracked or not the right design, or right colour.
We nearly gave up, but noticed a second bank of pots around the side which had just one pot the right size. But it was a duck egg blue and turquoise pot with a pleasing shape and a flat top (to maximise the mating area). I was concerned with its lack of rim (which I was going to use to mount the drive using some steel clamps)but decided that in the worst case we could return it or use it in the 'garden' (3mx3m patio).
Turns out that my fears were all unfounded. The driver fits snuggly but can be removed without scraping and sits so flat on the surface that I don't think sealing is needed after all.
(And yes, I use CAT5 pairs for my speaker wires. I figure if they are rated to 500MHz and 1A they are more than adequate for my <1W listening levels.)