That must be one custom mtn lite or its not a pogo chassis. Cant really imagine a 13.5" plywood base on a 16" wide tunnel not to mention the fact that at 22.5" your either not covering the foam and connecting the front and rear plastics or making it back to the reinforcements for the rear seat bolts through the tunnel.
If I had a measurable seat at the time, I wouldnt have posed the question nor searched the innerweb. But, for the real world, the phazer II is 16" x 28.5" and the phazers are 16" x 25". Both are 3/16" thick but 1/4" works fine. I think both could get by at 15 3/4" or 15 7/8".
Sometimes a person can stand back, read, and learn and sometimes a person can admit when they are mistaken or really dont have a valid answer. Even better, if someone really doesnt have any first hand experience or knowledge, just skipping on to the next challenge may be a better choice.