I just purchased this for the express purpose of a carry on for Icelandair:

https://www.ebags.com/product/eagle-creek/expanse-international-carry-on/342062?productid=10568859#ratings-and-reviews

You're right; the 7.8 inch dimension is really hard to find. I'm hoping 8 inches will work. Recently did a round trip on Icelandair and had the middle seat. Underseat room was mostly taken up by an electronics box. Don't count on a lot of underseat room!

By the way, no one was asked in either IAD, KEF, or TXL to put their suitcases or backpacks in sizers at the gate.