Judging from the photo on reply number 3, it looks like he cut the rubber seal on the vertical section about 3 to 4 inches up from the corner edge.
He did note that he only used 6 inch pieces of hose on each side, and to do that he would have to cut the rubber seal.
I did mine today with two feet of quarter inch air hose on each side and some dish washing soap.
I removed the black plastic gate that covers the rubber mat and black hinge, being careful and starting on one end and gently unsnaping the piece and then sliding it towards the tailgate to unhook the midgate side tabs, and I then also removed the black hinge, thus making it easier to remove and replace the rubber midgate seal so I could stretch out the rubber seal to insert the air hose into it, and it also allowed me to check-out the drain under the black hinge, which by the way was clean.
I then unhooked the rubber seal, layed it out straight, and put dish washing soap inside the rubber seal and on the air hose, and worked it in .
I didn't have to cut anything as two feet was long enough to make it pass the curve on the bottom corners.
I pushed the hose in about 3 inches further into the end of the rubber seal ends with a screwdriver.
I put a little dishwashing soap on the sheetmetal where the rubber seal attaches, and slide the rubber midgate seal back on.
It worked out real nice and looks new.
Now all I have to do is test it with putting some water in the bed and leave my front and rear sill plates off and check and see if any water runs down the channel underneath the sill plates towards the front of the floorboards where my carpet was getting wet in front of the brake pedal.
Bobby.