Increasing bulkhead thickness

I am doing an undercurrent system with some brute totes but the wall thickness is too small for 3" uniseals. I bought some rubber sheeting to increase the bulkhead thickness. My question is should the rubber sheet go on the outside between the manufactured head or the inside of the uniseal that expands? Also, the holes in the rubber were cut with scissors so the hole is only close to perfect... Hope this makes sense, I tried searching for an answer but no luck.

How thick is the plastic? I have used uniseals often and you must use a holesaw. There great but can be temperamental. I have also used on very thin walled containers as well. Use some Teflon tape wrap around 2 or 3 times and you should be ok. Also when installing pipe shave down the pipe edges to a point no blunt edges they can be a real pain in the ass to install

I'd say it's about 1/16th of an inch thick. I don't even think that's half the thickness of the required minimum for the uniseals. The hole in the totes were cut with a hole saw and are perfectly round and smooth. The sheet of rubber I had to use scissors with as the blades of the hole saw kept chattering.

I use actual bulkhead fittings from marine store and never have leaks but Ive only found them with barbed end. I also use ebb and flow fittings that are female thread.

Run the hole saw in reverse.... trust me you will be Abel to with ease