You may recall my shopping trolley trailer HERE.
It was a great trailer. Indestructible and lots of capacity in volume and weight. The only issues I had were that the cargo area wasn't weatherproof, it was a lttle too wide to comfortably navigate some of the areas I take it into, and it was heavier than necessary for the job it does.
After considering your comments and my experience using it, along with some productive dumpster diving it has evolved into the version you see here.
The plastic box I already had, its 2" narrower and 3" shorter (front to back) than the shopping trolley basket, its also a bit shorter vertically. However, its still enough capacity to get the shopping home, but most importantly the lid seals watertight.
The other alteration is the switch from 26" to 20" wheels (brand new out of a dumpster ) and relocation of the axle line approximately 3" behind the center of gravity. She looks a little rough because the welds need a cleanup, and the chassis a coat of paint. I really like the idea that the wheels are bolted not QR therefore a little safer from theft. Even with that very sturdy 1" square steel tube chassis she only weighs 20 pounds. total cost was a $4 spare tube, and welding consumables.