a clear, soft plastic connector with 3 or 4
ribs that takes a 38mm pipe and pushes into a 2.5"ish hole

That's it. They are polythene and I've had trouble getting them to seal. In
the old days they were rubber and just pushed in and sealed (some types had
a "lip" that folded round a boss on the pan). I've found I need to use
silicone because the "hole" isn't very smooth, the ribs are too stiff and
don't conform well enough, it can be difficult to get good alignment, and
they see some water pressure and movement during flushing. If you don't use
a sealant they can be OK at first but gradually work themselves out one rib
at a time. I've looked for rubber ones without success.