What about those rubber band kinda wrenches. I don't know the name but I have some. I do t know if you would get enough out of them tho. It has a rubber band and plastic handle...I use it to hold round stuff still.
I don't think I can get enough bite on it with the band type wrench as the nut isn't that thick. The spanner wrench would likely be perfect, but that's a bit pricey. Time to break out the jigsaw.
I've never needed a wrench to get a bulkhead to seal before. Try reseating the gasket and greasing the threads with some vaseline. You could also use the old millwright trick of placing a screwdriver against one of the nut corners and tapping it with a hammer. The screwdriver will bite into the plastic and turn the nut - just don't get carried away. Once again though I've never needed to do that to get a bulkhead to seal - only to get a seized one to come off.
What I've found when I started my drilled cube was that the seals had to be wetted before tightening and then it sealed. Mine leaked because there was an airlock under the gasket. If yours were dry, you may want to try that first.
dry seals will airlock almost everytime like 2wheels mentioned. any bulk head you should let the seal set just a minute in some water then rub it down and rub the area it will be sealing down. The water will squish out and it will seal nice. especially after it dries. good luck man