We prepared the lock walls with mounted steel plates (seals) where the edges of the metal gates are going to rest.
The whole point of a lock is that no water gets through - unless the gate is open. So that means you need to have a gate that fits very snugly into the width of the lock. If there is any gap, the water will find it! The engineering has to be very precise.
However, when we measured the lock with modern tools, we found that the old lock walls were not perfectly vertical - there was a tiny angle that left a difference of about 25mm (one inch) between the top and the bottom.
In the New Year we sorted this out by doing some very precise cutting of the concrete until the sides are perfectly straight, and then we fixed the steel seal plates into the walls. You can see us doing this at the half-way point in this timelapse film.