One idea that strikes me, and assuming that your primary intent is
separation for the purpose of preventing scratches, would be to eliminate the rubbers altogether and replace the hex-head steel nuts with nylon nuts.
I like your Allen Bolt Head solution better than what I am using for the two studs. Javad's bracket, which I bought and expect to arrive next week sometime will be used in product shots, but it doesn't feature the key-way. The Javad bracket is intended as a permanent fixture. If I make another one, I might explore using sheet aluminum for the bracket and the same studs as you've chosen, but I'll probably forego the hex-head nut and rubber washer. The other part of the mix for my next design is going to be how I get the fan arranged and mounted. The HPT435BT requires a fan. One solution is illustrated
here.