The KF-011 Kevlar wrapped cylinder is a good choice, but with a couple of drawbacks: It has a 15 year life limit from the date of the first hydro test before it must be removed from service, meaning after the expiration date, you will have a hard time getting it refilled at an FBO, you won't be able to get it tested and it might make your annual airframe inspection problematic. The second drawback is that it costs about 350% of a comparable aluminum cylinder. ($700 vs. $200 or so from Mountain High) But, if you can't find a suitable steel E cylinder, it might be the only choice that doesn't involve surgery on the aircraft to make an aluminum cylinder fit.