After making sure the part fits through the access port, I’d remove the mount assembly and work with it on its own.

With a properly shaped piece of metal, all you really need is a tap and die kit to create threaded bolt holes, plus perhaps a drill for the VESA mount plate. Our unique adapter ring system makes it compatible with any 21.5' and 27' iMac from late 2009 to 2020 (not compatible with the 24' M1 model). Our patented adapter simply clamps to the stock iMac base, and will fit any VESA 75x75 or 100x100 stand or wall mount. Not unlike the original iMac VESA adapter. VESA adapter for mounting iMac computer STAND-MACB from VIVO. Thus, all you really need to do is fabricate a short adapter that will attach to the mount assembly internally, extend beyond the shell of the iMac, and attach to a VESA mount plate. The mount assembly is much the same as the old one with the major difference being the aforesaid orientation of the set screws. Important note: Macworld and I take absolutely no responsibility for any damage you do to your iMac during any step of any alteration or use of a homemade adapter. I’m merely pointing out possible paths to VESA compatibility for those willing to take a risk-an extremely slight one if you’re capable. Alternatively, the brains to let someone who really knows what they’re doing handle it. This adapter is designed to work with a variety of iMac computers, features a brushed aluminum design, and supports the 75 x 75mm and 100 x 100mm VESA mount. Now I’m going to address the distinct minority of the iMac community who have the ability and gumption to disassemble their iMac, as well as fabricate simple metal parts. The bracket lets you attach your iMac Pro. Advanced DIY (or have someone else do it) The VESA Mount Adapter kit comes with everything you need to replace your iMac Pro stand with a mounting bracket.

Imac 2017 vesa mount adapter