So while doing some shower repair work, I learned that having a grout float would make the process much easier. Upon buying one, I noticed that they’re made out of gum rubber, the same material I make my robot wheels out of. For only $8, you get a nice 235mm x 100m slab of 15mm thick gum rubber! Depending on your wheel size, that should be plenty for a 4wd beetle! Once you factor in the shipping price from McMaster, this becomes an extremely cheaper option.
One small issue are these 2 extrusions on the top where the handle mounts. But these will vary depending on brand and size. Also they might not even matter for you depending on where you cut. In a pinch, I’m sure you could fill them too.
The specific float I used can be found here at Home Depot but you can find several different sizes and thicknesses online and at other hardware stores.