... EVA foam fitness roller (that's what it says on the wrapper, I swear!).
So, I was strolling through Aldi yesterday and I noticed these things for $9.99 and I thought, hmm...
I got the plain one.
Cut it into thirds, using an old tin as a guide.
Then split one part in half and the other two into smaller slices.
(clamping some poly pipe gives you a nice neck shaped dip)
Neck rests...
I suggest putting a cloth between them and finished necks as not all finishes get along with some types of foam. But, it could be a nice cheap set of neck rests for general work.