I think the resoning behind having the UAT vertical is so that any bubbles in the tank are at the top away from the area that the fuel is drawn out of the UAT, for the record filters usually have the inlet to the hollow side of the filter and the exit from around the outside of the filter medium.

If your tank neck is big enough try an Obit fuel filter or one of the paper filters that are used on petrol strimmers etc that usually stops any bubbles from exiting the tank on the way to the UAT.

Mike