Good morning and valid request. I have found the reproduction cardboard tank covers to be very rigid and lacking some creases, as well. I use a straight edge along the factory creases, as a break and overbend them a little in both directions to give the piece some flexibility. I then use a spray bottle with water in it, to mist the back of the cover, very lightly. I wipe the back with a damp cloth, to ensure uniform coverage of the moisture. This softens up the cover a bit and helps it go in.
I start by inserting the tabs along the top edge into the ribs in the cab panel above the fuel tank. Take care to make sure the ground wire from the fuel sender is properly fit into the slit cut into the cover. I then fold the ends, at the factory creases, and insert into the pockets at each end of the tank. With a short 2" x 4" block, I push the bottom of the tank cover into place, being careful not to tear the cardboard.
With some patience, a little moisture and softening up the creases, it will go in and look correct when you are complete. I have done several of them and they all turned out nice and stay in place, as the orginal covers did.
