I was missing the mounting plate as well, and couldn't find the exact mount that was used here.
Wanted to share what I did and list out the parts used for if anyone runs into this in the future, a google search brought me here after all.
1/4 inch Nut
Hex nuts are widely used in construction and to create a bolted joint. They are available with coarse or fine threads. Use with a screw or hex bolt to create a strong connection.
www.homedepot.com
1/4 inch x 3.5 inch Bolt (Could potentially do 3 inch, it'd be tight. see picture below. M8 doesn't fit in the VESA mount)
Hex bolts are hexagonal head fasteners with machine threads. They are commonly used for machinery and construction projects, both residential and commercial. Fully threaded sizes provide excellent grip
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12 Washers (See picture)
Flat washers are essential fasteners to keep on hand. They are used with a bolt or screw to evenly spread the load over a wider area. Flat washers also prevent loosening due to vibration, stop the bolt
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VESA mount
Buy Mount-It! Small TV Monitor Wall Mount | RV TV Mount | Quick Release | Fits 13 15 17 19 20 21.5 24 25 27 32 Inch Screens | 75 100 VESA Compatible | Low-Profile Slim Design | 30lb Capacity: TV Wall & Ceiling Mounts - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
www.amazon.com
Peloton Swivel Mount
Amazon.com: Selina Screen Swivel Mount | Pivot Arm Compatible with The Peloton Bike | 360 Degree Turn | Easily Adjust & Rotate Your Screen | Accessories by Selina : Sports & Outdoors
www.amazon.com
The VESA mount is attached to the back plate by two screws and two bars held together with rivets. Remove the rivets with needle nose pliers as well as the screws, you only need the VESA portion of the mount. You can see below how I attached it to the swivel mount. If you didn't want the swivel mount, I suppose it's optional. No reason you couldn't mount the VESA panel directly to the bike.
Hope someone finds this helpful!