When looking for Eternabond I did not see anyone selling small pieces, if I had I would have ordered one and thought nothing of it.
But as I looked around I began to suspect this is the same material used on my Lanai roof. Which does not cost $20 for a 4"X10' piece. Plus there are several others out there that all appear to be the same thing.
A little further search showed people using Peal&Seal with no issues. Plus MFM has been in business for 50 years where Eternabond has been around since 99.
So I bought the Peal&Seal will report back in several years on how it holds up.
Jim