The genuine vesa adapter from the company is out of stock everywhere. Benq tech support says the 3rd party company that made it went out of business and so Benq released some CAD files for the adapter. Using those, I created a printable part to convert Benq monitors with the same screw layout to a standard Vesa plate that can be used to mount the monitor to standard arms and other types of mounts.
I'm not totally sure which models are all supported, but it's been tested on the EX3501R and fits well. It looks like the XR3501 should also work. There may be other monitors that work as well. If you have another model that works, please let me know and I'll add it to the list.
Jul 27, 2018 Edit: Modified the model so that the extra holes on the monitor side are drilled all the way through to accommodate the little nubs on the monitor.
Aug 7, 2018 Edit: Laid the model flat on the bed surface for easier printing.
Since this is a crucial part supporting an expensive monitor, I suggest printing at 100% infill just to be safe. If you're comfortable printing with less infill, it should still work, but it's not been tested how well it will hold up.
Use this part at your own risk.