Stupid noob bulkhead question

[Coasting]

Okay quick question folks!

I'm trying to install the bulkheads on my little Aqueon Evolve 8 tank.... Since I drilled holes in it way back when for the nano tank contest so I could plumb it for a sump, I haven't touched the tank since and now intend to put a betta in it I have to plug the holes....

Uhm I get that the rubber washer goes on the wet side of the bulkhead but one side of the rubber is totally flat while the other has 2 circular ridges in it I guess you would call them. Which side goes against the tank? I'm doubting it matters since both the bulkhead and tank are plastic and acrylic and totally flat. But just thought I would check before I have water all over the floor of the place I'm renting
Thanks