It's the correct material, it's pliable yet strong. I'd have to measure my box when I get home.
As described in the build cut the rubber the same OD (outer diameter) as the box face. Rough up all four sides to give them an even look.
Remove approximately 1/8" of rubber, approximately 3/8" to 1/4" at one end. Use a belt sander or similar. Remove around the circumference of the rubber so that the rubber sits firmly recessed in the box face when finished. This will give the box face something to be glued to.
Afterwards drill the two holes for the 3/4" chicago or similar mounting screws, as described in my build.
Once the holes are drilled sand a 3/8" contour into the opposite (flat) side of the rubber, lengthwise. This section will mount against the floor mat material of the waist belt without flexing too much and causing it to pull out of the front box face.
Once that is finished insert the screws and check the fitment against the belt. You should have enough screw housing protruding from the rubber to go through the floor mat material & neoprene spacer of the belt itself. Double check the male and female threads to insure you're not going to have assembly issues later on. If all checks out ok then remove the screws, add glue to the head and shank (E6000 is what I used), install in the rubber and let dry.
Once dry Glue the box face to the rubber.
Once dry mount the assembly to the belt.