Bottle mount battery wobbling

I'm using this Reention jumbo pack on my WIP townie conversion (will make build thread when done).
The fit is excellent, and it matches the bike perfectly too, but it's wobbly, only laterally though. Sure a little wobble is essential to soak up shock and vibration but I'm afraid this is a bit too much wobble.

Judge for yourselves

Under the video there's a picture including the mount, you can see it's part plastic and part metal. The battery slides on and latches onto the plastic part, and only the plastic part is flexing, the metal is much more rigid... obviously.

How can I improve the lateral stiffness?
Thanks in advance

Betcha that's going to rip the rivnuts out your frame,.

I would mount the cradle on a metal plate. Screw the plate to the bottle holes for alignment. Add a pair of u-clamps to keep the plate from moving. I had a little different cradle, but the thoughts are similar,

The solution sort of depends on what kind of conversion you are using. Front hub motor, rear hub motor or mid drive.

But first: a man's Townie has a lot of space for hanging the battery in a bag suspended from the top tube. The bag with some bubble wrap under the battery will provide cushioning that protects the battery from vibration fatigue. Straps to the down tube or a brace from the top tube to the down tube would prevent lateral sway. The battery placement fore and aft could have some room for adjustment. I'm pretty sure there are bags meant for this purpose, though maybe not for the Townie's curved top tube.

Front hub motor bikes balance well with the battery in a bag on the rear rack. I use a plastic cutting board under a Harbor Freight bag for this purpose. The battery I used was a 48v, 13.5ah Shark, it looked like it would be unstable on the down tube or seat tube, like the cradle would eventually break, but sitting on a cushion in the bag it was never a problem and it was protected from the vibrations that frame mounting would have. This arrangement rides very well. For a lower center of gravity, the battery can be carried in a pannier bag on the side of the rack. That's how we do it on my wife's Townie, front hub motor, battery in a pannier on the side of the rear rack. Top rack bag has the big advantage of easy and fast on and off battery mounting. Also, the 15" Harbor Freight bag can carry the battery and one or even two 2 liter soda bottles. The controller can be mounted on the bottom of a rear rack, off to one side.

Rear hub motors with the battery on a rear rack are different because the CG is too far aft. They ride well but can get out of control when stopped, especially on an incline and especially on uneven ground when dancing for footing. I dumped my full suspension, rear hub motor bike when stopped a few times. This instability happens when the riders weight is removed from the seat, pull on the handle bars and the front wheel comes up off the ground and swings to the side. Now the battery is placed on the side of the frame tubes, where the open triangle would be on a non FS bike. As above, a battery can be suspended under the top tube of a Townie. Surprisingly, factory Townies with rear hub motors mount the battery on the rear rack.

Mid drives should do well with a battery suspended under the top tube or mounted on a rear rack, either top or side in a pannier.

800 miles ago I switched from a crank forward bike, much like a Townie, to the full suspension bike. I miss the crank forward riding position it was more comfortable and gave better leg extension while still allowing good foot contact when stopped.

Hope this helps, keep us posted on what you do.