I have done something kinda similar using an old VCR as a case. I stuffed a motherboard in there and used plastic standoffs that had collected in my box of computer crap. I ended up gluing a few pennies to the bottoms of the standoffs to make sufficient clearance under the board. I figured out where it needed to go and then glued the bottom of the standoff/penny stack in place.
I just looked at the link you posted and that is a full size ATX board. I saw 3 different case sizes for that Optiplex 960, but it’s going to be hard to make any of them work. You won’t be able to have the PSU inside the case with that beast of a motherboard crammed in an mATX case unless you do even more hacking on the case. At that point it would be less work to create something from scraps than to disassemble and reassemble a backwards BTX box.
Speaking of boxes, you might seriously be better off using a cardboard box than trying to shoehorn that thing into your case. This Is My Budweiser Box Case that wasn’t pretty but did it’s job. I have also run computers on top of magazines, plywood, or in the box they came in. It’s perfectly fine until you can hunt down another case on the street that better suits your motherboard.