When are you going to build it?
You dont have to use a Tee but the Tee is best if you plan to change the angle of the shot. The cannon i built in the carriage has a fixed angle so the water wont spill out it just sits down in the corner (cheap design). The Tee holds the water in a little spot below the chamber and it can be tipped back and forth some what without spilling the water out.
The first few i built i put cleanout caps on both sides, i figured ide want to "clean them out" now and then. I found it just made more of a headache because the bottom one ALWAYS leaked if i left it loose enough i could unturn with my hands. The last few i built the water sits in a cap glued using the slip fittings so i pour it in from the back and then sort of forget about it.
I watched one youtube video this guy built a carbide cannon and his design was a little different. Not like a cannon but IMO setup to be a little easier to use and cleanout. I havnt built one yet i had all my 4"-3" built before i seen it or i would of built one like his.
He used his tee 90° from the way i normally use mine. He dumped the water in, and the powder in from the top with a small spoon. The ignition could also be on the cap too to repair or replace with a backup cap. I thought it was clever
My traditional build (copied off youtube plans
https://www.youtube.com/watch?v=OUuwCsNL1lI) top, his design on the bottom.