RE: Doesn't look like it can be converted
looks good! nice to see that you have access to all four case screws for mounting. i've done up a similar mount long time ago and was pretty easy. most important part is that your spacers/stand-offs have enough diameter at the ends so the whole thing will be rigid. at the case you're limited but at the mount end you can have a nice big diameter with lots of surface area.
my stand-offs were drilled and tapped on both ends. one end threaded onto 10mm longer case bolts and the 5mm alum plate mount went on the other end with countersunk flat-head screws. could also just forget the mount plate and use screws from the backside of the firewall into the stand-offs. don't bother with the rubber mount, just added weight IMO.
dave