Since your e-scooter has dual steering tubes, I would suggest a lock-holding accessory that attaches to both. One complaint I have with similar lock accessories to the one linked is that they have the lock – which necessarily is made of solid metal – cantilevered out, resulting in a large moment arm. This means during sharp bumps in the road, the unsupported end of the lock will apply a torque to the plastic accessory, which inevitably ends in fatigue and breakage.

Some of this is down to necessity, because there’s no great way to hold onto a U-shaped lock at just one point. And also because the stock accessory for U-locks use this poor design. But in your case, you have two vertical tubes that can spread the load.

I think the ideal accessory in your case would be shaped similar to a wide halyard cleat arranged vertically, so that when the U-lock is opened, the U portion can be slid underneath the lower cleat and then locked to the top of the lock, which slides underneath the upper cleat. The act of locking snugly to the cleat will pin the lock in place while preventing cantilevering and noisy wobbling. Sadly, I’m not aware of such a ready-made design, but perhaps that’s something that a 3D printer enthusiast can help draft.

As for the cargo trailer idea, be aware that weight-and-balance need to be taken seriously for smaller configurations. I’ve written about trailers before – although for moving bikes behind a small automobile – and this is the relevant section:

Even if tongue weight were fine, the arrangement of the bikes on a hitch carrier have their own impact: moment of inertia. That is to say, if six bikes were arranged parallel to the drive axle, they’d have to each be spaced at least 8 inches (20 cm) apart. For six bikes, that means the carrier extends beyond the rear bumper by 48 inches (1.2 meters). And that’s just too much inertia to exert on a small car’s hitch receiver. Any bump that the car drives over would cause the bikes to also bounce, but that bounce then reflects itself due to the long moment arm, which then affects the car again. This is an identical wagging-the-dog scenario as trailers have, but here it’s a vertical wag rather than a lateral wag with a trailer.

In your case, imagine if you had a trailer and then came up to a speed bump. The front and rear e-scooter wheels have suspension, so they can absorb most of the bump as you roll over it. But most trailers don’t, so the trailer will jump up at its axle. If the center of gravity (COG) for the trailer (including payload) is ahead of the axle, then the axle rising will cause a downward force at the hitch attachment to the e-scooter. This is generally desirable, but the hitch and rear wheel must be able to deal with this.

But if the COG is behind the trailer axle, the axle jumping up will cause a lever that tries to raise the hitch into the air. If it’s very forceful, it’ll lift your scooter’s rear wheel up too. This could easily lead to a crash, since all two-wheelers need two points of road contact to be stable. The physics is identical to automobiles pulling trailers improperly, either due to driving too fast or badly loading the trailer.

I don’t want to discourage the idea, but make sure your trailer is properly attached and properly loaded. The reality is that automobiles and trucks are more forgiving because they have lots of mass. But two-wheelers are a lot lighter and need careful attention, since upsetting the wheels will also deprive steering control, which topples all two-wheelers.