Fear not fishboy, the ice-cream made it safe and sound!
Just back from another shop as it happens – additional bag strapped to the top this time for more volume.
Bigdummy, the bag straps drape over the ends of the frame and then buckle underneath so the thing is tensioned. Not completely waterproof, but I’ve added some seam sealant to obvious weak points.
The bag comes off the frame and can be worn like a courier bag. The routine we’ve got into is to take the bag off the trailer on arrival at shop, lock trailer to bike and rack, carry bag round shop while shopping, put stuff through checkout and straight back into trolley (no carrier bags), take trolley out to bikes, strap bag back onto trailer and then pack shopping.
I reckon the weight today was 20-25kg. Handled fine – I ride a singlespeed at the moment so honking up the hills was required and no problem. More fun and less stressful than taking a car (we sold ours a wee while back).