AFAIK boats had a D-shaped rudder immediately aft of the prop, with the tillerhead going down through the stern deck whereas buttys had something altogether more elaborate curving over the transom. Good pic here:-
While going ahead you need only something to divert the prop wash (look at the sort of thing they have on seagoing mobos). When reversing however you need something a bit bigger, preferably partially balanced to stop it wanting to slam hard over. Some narrow boats are easy to steer in reverse and others are complete sods. Best try a few and see what works.
You definitely need the skeg shown in the drawing above, as the canals are full of rubbish. It will also give support to the rudder in case the rudder catches on the lock sill when locking down. All too easy to bend the rudder post otherwise.
They are simply cut out of mild steel plate, say 10mm.