In the pictures below, the chassis is lying on its side, so up is to the left.
The 1st picture is the installed backplate.
The 2nd picture is the backplate being removed. You can see that the top screws go through the chassis and screw into the backplate, while the bottom screws go through the backplate and screw into the chassis.
The 3rd picture is the MB699VP-B just sitting in place, but you can see that it fits nicely. What it would take to mount it is basically a backplate that was similar around the edges, but had a sleeve that went inside so that a drive could be fastened there.
But, then, I took a picture of the inside, and that showed me the way I could make this work:
You can see that there is a flange that would hold the front of the drive, and another flange sticking up from the motherboard tray that will hold the left-side rear of the drive. Those flanges are for the version of the chassis that a uses regular ATX power supply.
The cable management tie that I had stuck to the bottom of the power supply sleeve gave me the idea to use a pair, and screw through them into the right-hand side of the drive. I could use a metal strap, but despite the holes in the power supply sleeve, you can't fasten anything using them, as there is zero clearance inside. So, I'd still have to have some sort of "stick on", and I've already got one that seems to work.
The last thing I am going to do is put another stick-on behind the drive on the motherboard tray. This will keep it from sliding back and forth as much.