Wooden rings are nice. I've made several pairs, for myself, friends, and for sale, and they've all turned out at least decently well. Unfortunately, they aren't very tactical.
If you want instability training for the legs, try a slackrope
. Just 40ft of 3/4" manila from Home Depot will do, and it only costs about $40... Add two trees, tie two timber hitches, and you're in business.
(Note: You assume all liability for injuries sustained while trying to walk on a rope. Though it's pretty hard to fall and not land on your feet, as long as the rope is 1-4ft from the ground. But yeah, it's still your decision to try it or not.)