SHOO-GOO gets my vote in front of any other RC shell flexible type repair type stuff...
it's the bizzo....! 'n I should know, I use gallons of the stuff with my standard of driving... (Hhmmm...schmells good too!!! not that I condone ANY type of solvent fumes inhalation of course...!) You can even repair shoes with it...! (no you can.. really...!)
Sadly, it can also add weight to your shell... more than you think it would!!