I use a set of motocross shin & knee protectors on my arms as forearm & elbow guards. They work great. Plenty of protection, far better than my SPES elbow cups + forearm.
I can move just as well with or without them on, and they are lighter than the SPES combo.
One issue is that the clasps broke. I fixed that with an elastic strap and some Velcro. At the time I noticed it it was an emergency and I just taped them to my arms.