I rob my toilet for the poly tubing. Fluidmaster 400A toilet valve always provides extra long tubing for the overflow... and it measures .275" OD x .040 wall, yielding .196" ID. Just cut off a couple inches from your existing toilet overflow and you're set.
If you can mill the recessed seats as previously suggested using a 17/64" milling bit, you will have a larger diameter TP that will make an excellent seal, providing more volume in which the CO2 can expand behind the pellet. More volume is a good thing with CO2.
Here's a tip for cutting any polytube square... Whatever size tubing you decide to use, feed a short length of it snugly over the shank of a drill bit of the appropriate diameter. Then chuck the bit into a variable speed hand drill, flutes first. (I do this in my lathe, but the handheld drill works just as well.) Using a sharp X-Acto blade in its holder, score the tubing with the blade as the tubing is spinning. This will make a nice, square cut across the tubing. Make a second cut at the length needed (it'll take a couple tries) and you're done.
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