It doesn't have to be that strong Peter, I would just use two or three layers of chopped fibreglass matt brushed on with plenty of resin over the underneath of cracked areas. If you have to repair an open hole it's best to apply just one layer and let it go off, leaving a stiff base to add extra layers. If needed an extra layer can be brushed onto top surface but it would have to be thin to not compromise the space the jack fits in.
Spraying the finished job underneath with matt black paint will leave it looking neat. Your suggestion for the upper surface flock looks good but you will likely need to cover the whole area to match.
