A flange is normally a thing that flares out from the size of a pipe, to something at right angles to that pipe. It allows you to connect it to another flange with bolts.
It turns out, the thing you have in your shower around the drain is a puddle flange.
I found one that has a hole in it that perfectly fits a 50mm PVC pipe.
I'll stick a 50mm pipe into the hole to act as a standpipe, and silicone the puddle flange into the hole that's in the bottom of my new grow bed.
Because I bought a pump with expansion of my system in mind, it's too powerful to need to run it 24/7. I'll be putting my pump on a timer, and running it for perhaps 10 minutes per hour. This means I will probably settle on using a slow leak approach rather than bell siphon approach to drain my grow bed.
The slow leak approach will involve putting a hole in the bottom of the standpipe, just up from the bottom, so that in time all the water will drain out. The standpipe is there as an emergency overflow pipe.
If for some reason the water fails to drain out of the grow bed, on the next cycle, the grow bed might overflow. With a standpipe as an emergency overflow, the grow bed might remain flooded, but the water will overflow back into the sump.
I'll drill a small hole in the standpipe to start and see how long it takes to empty the grow bed. If I need to, I can then drill larger and larger holes, until the grow bed drains over the space of about an hour.
I'll also be setting the drain up so that if at some stage I need to change the system to a bell siphon based one, all I'll have to do is replace the standpipe, and add a bell.