This is what I have done to my supergoose, I've had 4 mongoose frames in total, all of them had slight crushing of the clamping area.
BITD as kids we didn't really notice or care and just crushed the clamps until the seat stopped moving and this is why most of them are split now, the seat post tube on a mongoose measures 22.6mm and a standard seat post is 22.2, 0.4 of a mm is enough to split or crush the key way when tightened
I bought a USE seat post shim from wiggle 25.4 and cut down the length so when fitted around the seat post it has a slit the same width as the key way in the frame when fitted, the shim is 0.4 mm.
2 of my mongooses also had slight weld showing through from the top tube inside the seat post tube,(worth checking on any frame) this would make a 22.2 fit tight in the tube but only in that raised area, the rest was loose so i carefully ground the high spot so the tube is the same all the way down.
Better than the coke can trick and it has a nice lip on the top so you don't loose it when fitting the post.
I fitted this because a hutch layback only comes in 22.2 or you can fit a hsin lung post like i have on my fatgoose (pictured below) which also come in 22.6mm standard
You require a digital vernier gauge to get it spot on though
From BITD if I remember some Redlines were supplied with a 22.2 seat post and a shim from new.
