... 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.
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Cut it into thirds, using an old tin as a guide.
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Then split one part in half and the other two into smaller slices.
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(clamping some poly pipe gives you a nice neck shaped dip)

Neck rests...
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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.