An elephants trunk is the common name for the drain tube fitted into the low part of the transom on your RIB
The round tube that is glued into the transom is ~68mm o/d
There are two jam cleats on the red card in the picture ready to install on the transom for the rope to be fixed into whilst the boat is in use and the drain in the up position
These items are not cheap - They are complex to make and time consuming