This looks really interesting. The only thing I don't like is having to cut into the bottom of the fiberglass to make it fit. I rather have something that just screws or bolts to the bottom.
jw9800 everything about building one of these is custom cut and fit. Nothing is just "bolt-on".
There is a "method to the madness" on how and where the rollers are located.
Three rollers: The front of the bumper is curved. If you just put two rollers at the ends of the bumper, the center (being curved) would contact the pavement before the rollers get there. The center roller will prevent that. It might be better with five rollers but I drew the line at three and careful driving.
Size of the rollers: Rollers too small will not handle the job. Rollers too big will be way too obvious and hard to mount.
Roller mounts: The rollers are placed half way under the bumper because that is what part of the roller will be used and that allows you to use a large roller without too much showing. The top of the roller just returns to share it's turn. You place the roller as far forward as you can so they will contact the pavement before the bumper. That means you trim the underside of the fiberglass bumper to get them into position. Nobody sees the bottom of the bumper. Not too many people will see the rollers.
You can't be afraid to cut and trim to make your build the way you want it to be. The rollers will prevent unsightly damage. That is worth the small effort.
Other people have done different things to deal with the bumper scraping. Maybe you would prefer another way.
RT
Also, these rollers have to support the weight of the front of your car. I would not rely on the fiberglass bumper for that so don't just bolt something onto the bumper. Make a steel support structure for whatever method you use.