I have two flat Hurst shifter handles like the one in my picture. One in my bench seat 62 Hardtop with the round boot and round hole in the boot, the second one in my 62 SS 409 hardtop with a SS rectangular shift plate boot with the round hole in the boot. Both have been in the cars for over 10 years no rips or tears. I think both boots are them are Soft Seal boots. The fit was tight when I installed them, and the handle only is in the round boot holes(none of the bolt on handle portion of the shifter is in the round hole in the boot).
The only real way to install the boot and retainer is to cut the carpet just large enough to mount the boot ring over the boot, since the carpet fits under the boot lip that covers the retainer. (this is especially true for the bench seat round boots.)
I have a very sharp scissors and also use a carpet cutter/blade. I do all the cutting without seats in the car.
You need to cut away the insulation on the bottom of the carpet to help with making the hole. Another help is to burn holes (carefully so the holes aren't too big ) in the carpet for the mounting plate screws with a soldering gun(this keeps the carpet loops from wrapping around the screws and helps you find them screw holes.
Paul