I have the Abel Armrest. It is well made, but for whatever reason, I can’t get it to fit securely on my guitars.
I am a huge fan of the hard to find Luva sleeve. These are very comfortably padded, feel completely natural, and do no damage to the guitar whatsoever. I find a significant benefit in terms of tone production, right hand speed, and lack of fatigue, due to the absence of forearm muscle compression that is caused by the edge of the guitar. An added bonus is protection of water soluble French polished finishes.
Since you simply slip it over your forearm, they are instantly adaptable to various instruments.
I don’t know if they are still being produced, but are still available at GuitarWeb-Shop in Switzerland, I believe. They are delightful people to deal with, and shipping to the US is no problem.