Enclose in ceiling speakers

If u don't see one in partsexpress.com, it's unlikely u will find one.

You'll first want to find out from the manufacturer what the recommended box volume is for your speakers. If you can get to the speakers from the attic, you can cut boards to size to box in the specified volume using the cieling joists to help form the enclosure. If you can't get to them from the attic, you'll need to access from the room side which means dry wall repair or purchasing something like this.

-Brent

Why do you need an enclosure, for protection on the back side? If the speakers are off the shelf ceiling/wall type they are likely made to operate in IB (infinte baffle). Putting those type of speakers in a small enclosure will serve no purpose other than maybe raising the power handling a bit and raising the FS which will limit the low end of their frequency response.

No need, if anything a little owens corning to limit any echo getting back in the midbass driver...
They should be XO at 80hz, better 100hz if you can

Yup, you would do it anyway. I've done a few installs and that would do it

I don't think Sonance or Polk directly mention back-boxes in the sales literature for their in-wall/in-ceiling speakers either, but they sell them as accessories. There are probably other brands they go to the trouble as well. I expect most in-wall/in-ceiling drivers would benefit from a properly sized enclosure, but the percentage of people who would actually install them is small so most mfgs. don't bother.

Anectdotal data point. A few years ago, I installed a pair of HiFi Works (division of Sonance) THX certified in-wall surrounds. I emailed Sonance/Hifi works for the recommended back-box volume. After some use without an enclosure, I went ahead and boxed in the joist area around the speakers to the recommended volume. It made a significant difference in the bass response of the speakers. Unfortunately, I didn't take any before/after measurements so I can't quantify the actual improvement. IIRC, the enclosure dropped the Audyssey recommended xover point 20Hz or more.

Similarly, I've since picked up another a pair of the HiFi Works THX LCRs, although theyr'e not installed yet (future project). The installation manual for these actually recommends assembling a back-box around them for best results. And, surprisingly, since one would expect in-wall/in-ceiling woofers to be optimized for infinite baffle operation, HiFi Works recommends only .26 ft^3 for these...rather petite by IB standards, IMHO.

-Brent