The short bed tray gets the job done if it's your permanent solution. Be prepared for a frustrating install.
If you plan on switching out the shortbed tray for the child seat or any other accessory, then I would not recommend it.

Positives:
* It's nice that it can be mounted widthwise or lengthwise.
* the bungee straps are great
* It looks good and is sturdy as one would expect.
* Works on my HSD unlike the website description

Downsides:
* This is a REAL PAIN to install. It's so difficult to access the underside of the frame to secure the locknuts. Plus, you have to balance a washer on top of the locknut without it falling off while trying to align it with the screw. I had to use tape to hold the washer temporarily. Then there's the plastic washer that sits in between the frame the tray. Those are tough to align all 4, though, I found a way through trial and error.
- Manual: https://live.ternbicycles.com/2024-04/24-TN-UM-Shortbed_Tray-m_05-web-v01.pdf?VersionId=3.VysNkFszZYKT29Rp6_OVVvAfaWg9jj
* It's SO MUCH OF A PAIN to install that it's only worth getting if it's a near permanent attachment. Do not plan to switch this out quickly and easily.
* The flat insert is a thick piece of sheet metal! It adds so much unnecessary extra weight to a bike striving to be as light as possible. Plus, you have to install 16 foam separators in order to prevent metal on metal rattlings... but it still wasn't enough:
* The shortbed tray frame has a ton of weld points and they weren't all flat enough to prevent the metal insert from making contact and rattling! I had to file down some of the welds in order to give enough clearance to the metal insert to not rattle.
* Minor issue, the instruction illustration for the 16 rubber pad placement is incorrect and mirrored
* Did I mention it's a real pain to install? Luckily, I had a bunch of different tools and ended up using my needle nose vice grips in order to barely grab the locknuts and hold onto them. If you don't have many tools, then prepare to buy some just for this

I expected better from a company that touts all the accessories and has been making cargo bikes for a while. I'm disappointed with this mounting and design choice when they control all aspects of the design.
Still, I need the shortbed tray for my errands, so all this one time effort is still worth it to me.

The shortbed is well made and comes with bungies, which was a pleasant surprise. It’s stable and doesn’t rattle, thanks to the rubber strips and plastic washers included.

The downsides are that it’s relatively frustrating to install due to the number of small washers used and the virtually nonexistent clearance under the rack for your hands. I was hoping this would be something I could take off easily to swap in the seat pads for my kids to ride along between grocery trips and deliveries but, honestly, it feels like a semi-permanent addition given the amount of time and effort required to install.

I’d gladly buy another if it had something approaching a quick-release system.