100% custom. I didn't like any of the pre-cut options: I wanted something that followed the body lines, and I really prefer to make my own stuff whenever I can. I worked a little in a body shop back in the day and I picked up a lot of little tricks for this kind of stuff.
The foam is actually Hydro Turf (mostly marketed to jet-skiers). I sketched out a template on posterboard, cut two mirror image pieces and smoothed up the edges with sandpaper. To attach them, I used 3M headliner adhesive - designed to make foam stick to metal. If works excellent if you follow the directions... clean everything beforehand with alcohol, do it on a hot day, or use a little forced air heater to preheat the metal like I did... I bought a bunch of cheap magnets from Harbor Freight, pre-placed the pads how I wanted them with the magnets and then went around the perimeter with masking tape and masked off all around. Sprayed the tank and the back of the pads with the 3M, and then used the masking tape perimeter to line them up just how I had them. The down side to the headliner adhesive is that it's a contact cement, so you get one shot at getting it right or you're basically just going to have to clean it all up and start from scratch. I used the magnets to hold them tight and give the adhesive time to cure. After that, cleanup was mostly just a matter of peeling the masking tape. I'm really happy with how they look and how they work.