I thought of using one of those hi viz dog jackets (in cat size obviously!) for Balders as he's black and his eye doesn't reflect light. But I decided he'd be a liability with it, even more than he was with a collar on.
TBH, short of keeping him in or walking him on a lead (always good entertainment value for the neighbours) I can't see a foolproof way of keeping a cat safe. Cat proofing the garden, of course, but that's out of the question in this case (and is with us too). Making sure he's in before it starts to get dark will help, as will going outside and spending time outside with him - we keep an eye on Balders when he's outside so are able to get him out of some trouble.