Drainage when washing inside canopy

Get a hole saw kit if you really want to go up to 25mm. There is other alternatives, that might blow your budget.

Bob

Do you need to wash out the canopy from carrying a particular product because generally just a wipe over with a damp rag works.
Step drill bits come in various sizes and still can be useful for other jobs after you drill a large hole in the canopy.
Or you could fit a locking cat flap.

Mike.
If you must wash inside, park so it drains well to One corner and simply use a Wet Vac to suck up any needing removal. The wet vac can also be used to suck dry corners, laps and crevices otherwise holding moisture.