UDATE 2019-10-19: Added faceplate for Maestro/Ethernet, which connects with 2x M2x15 bolts. I will add a faceplate for the WiFi soon.
UPDATE 2019-09-17: I put a skirt around the risers and increased the size of the duct opening to improve airflow over the drivers. I also added a MOSFET mount, since the fan shroud displaces it. I think this solves all problems quite elegantly. Now...chop a hole in the side of your control box that clears the vent holes by 1mm and the 2xM2 screw holes to either side by 2mm. Faceplate incoming in the next week or so. Old version deprecated.
Special thanks to @tonypiom for alpha and beta testing this mount.
UPDATE 2019-09-16: It's been brought to my attention that this mount displaces the MOSFET with no offer to accommodate it, and every version of it has been plagued with weak standoffs due to print orientation. Atop that, I have concerns about airflow to the underside of the drivers (where they are cooled) given how much higher the board sits on this mount compared to the one I made for the Ender 5. As a result, I'll be uploading a new version of this tonight (probably, I want to install my Maestro, and this is already putting that back another day) that brings a skirt up all the way around the board, and shoots a mount for the MOSFET off the side of the fan shroud.
This is an adaptation of my unpublished Ender 5 Duet Mount, modified to work with the Ender 5 Plus. It uses a 40mm fan (recommend 24v SUNON 4020 or 12v Noctua 4020 via a buck converter or another fan somewhere else in series) to direct air under the board, which escapes through 3x3mm vent holes in the front of the mount.
There are 2 M2 holes centered and spaced 80mm apart that I will mate a faceplate to for the Maestro. It'll also work for the Duet 2 Ethernet, as well as the WiFi (but with a large hole for the WiFi module).
I ran a lower-to-the-floor version of this mount (necessary for ethernet port to clear the 2020) on my Ender 5 for months with zero issues, and my board stayed nice and cool, even for prolonged periods.
The only thing really different about this one is it's the original height with original height standoffs, and I removed the second fan mount (as my 5 Plus has dual 120mm 12v fans cooling the PSU and board). I also had to move the mount holes for the stock board towards the edge a ways due to the slope of the control box.
I print all my parts solid. This doesn't need to be incredibly strong and doesn't have any threading, so use whatever resolution and infill you want, but give yourself at least 3 walls.
Print with supports from build plate only to support the bridge under the duct opening.