LoL, they do make the roll on garage floor kit lol.
How thick do you need it? Because you definitely don't want the spring to get so loose when the car is raised that the spring falls out of the upper mount "cup". What you need is for something to secure the top of the spring in the cup because placing something on top or bottom of the spring will just end up raising your car. With that, you might as well just adjust your lock rings higher then, but I know that's not what you want to do.
One option is to order new bilsteins with a shortened stroke.
Second option is to somehow fasten the top of the spring into the upper mount "cup". My idea is scary to some but I'd get a dremel tool and cut a tiny slit in the mount in one spot towards the top on the side, small enough for the narrowest hose clamp (tights with a screw driver) and secure it that way. You can do it on the side facing inward on the car and no one would see.
You can try a bracket and screw it to it also but that has a risk of screws coming lose and dropping the little bracket.