For long I postponed getting myself a proper footrest and used whatever I found around to rest my foot on while recording guitar. Finally I did and I'm glad I did. During lengthy recording sessions it makes a big difference how you position your body.
You can adjust the height of the footrest easily and it's also angled, so that it's comfortable to use.
It folds down completely flat and it's quite compact, so it's easy to take with you.
It might be a bit flimsy for rougher use, but I've had no issues with it during countless hours of use at the studio.