I have a hot knife from Harbor Freight, "foam hot knife" might be an easier search for you since what I have and links won't do any good in Europe.
The long bladed ones tend to bend to easily; mine is a little under 6 inches, the upholstery place I know of uses one n the 5" range.
Depends on how many segments you need to have...2 or 3 and how many you are going to make if a hot knife is a good investment for your project.
You can buy rolls of the stuff without needing to cut the entire pad out, and most sun pads are 1-1.5 inches. A serrated blade should do fine for cuts that are just the width of the pad (electric or not), or a band saw will do this sort of cut quickly if you are only needing a few cuts overall.