Last year I fitted a toolbox under the rear side panel. (You can read about it here).
It carried a couple of bottles of oil and a couple of bits to France. When I got back one of the supports had snapped on the box, but it still held.
When I stripped the LI for an all-over service, I wanted to remove the battery from the legshield toolbox and move it to the back of the Lambretta. Dave Mayo May who has an FB page LamFab Scooter Fabrications. supplied his version of a battery tray for under the side panel.
It comes in Stainless Steel, in two parts. That means you don’t have to unbolt it from the frame strut to remove the tray for wheel changes etc. There’s a thumb screw that holds the tray to the support. (I used a bit of Loctite for added security). There’s also a Velcro strap and two Nyloc nuts & bolts, which bolt it to the rear frame strut.
You also get some sticky backed foam padding to absorb any rattle from the battery.
As with all these rear fitting trays. Always make sure there’s clearance from the rear hub.
It’s been fitted for a while now and there have been no issues at all.