Cutting all the free flooding slots is pretty tedious. Most are on the upper hull but their are quite a few, forward and aft, on the lower hull.
The hull on the Engels is fibreglass. For each slot, I drilled a row of holes, joined them together with a round needle file, then filled to shape.
It is time consuming, I found the drilling worse than the filing !!!!
if you want a static diving sub then you can't get round doing this chore.
If you are OK with dynamic diving then look at the new Robbe kit. The slots are preformed