Yes, the 3-ft long black or gray cylinders. It's already pre-slit longitudinally, so you just slide your finger along the slit to free it up. It springs right back to the original shape.
It's sized for either 3/8" or 1/2" pipe, and I used the 1/2" (ID) size. It's fairly flexible, retains its original shape, and has a lot of friction on slick surfaces like lacquer finishes, so you slide it on and it mostly stays put. It slides right onto the edges of the tailboard and bass bin top plate. I didn't need adhesive, and didn't want to anyway, but, yes, the walls help hold it in place.
I used a utility knife to cut angles and notches so it would stay in place without gapping. For example, I think I had to cut notches for where the braces attach to the tailboard, and angles where called for at the ends. I think I cut separate lengths for where the tailboard steps in for baseboards. I also cut little plugs for where wire goes through holes in the bass bin top.
The AK-5 change made them sound a little hollow, like there was a small dip in an important mid-bass/midrange area. Maybe I was used to the AK-4 config, but I was glad when I switched back. It's an easy change to make and reverse as I recall, but don't break a wire at the end of a cap.
BTW, I've always put that insulation on hot water pipes wherever I've lived, because it keeps "hot" water hot or warm quite a bit longer, a nice feeling, believe me.
Hope this helps.
Larry