Here is how we did the Matco booster spring.
The spring is a Hillman #181 (3 1/16" x 9/16" x .054) but others will work I'd imagine. At the bottom is a nylon 3/8" shaft bushing. The set screw shaft COLLAR holds it all together. Several RVers on the field are using this setup.
I went with the Grove master cylinder since I hadn't used my Matcos but the springs are definitely cheaper.