I think I’ve finally found a plastic crate in a practical size that can be sourced locally (Australia)... and I wasn’t even looking for one.
Size-wise they’re a touch smaller than the big Wald 159 (by a few centimetres wide and deep, the Wald basket is W53cm x L38cm x D23cm and these are W50cm x L36cm x D27cm (internal) and 50l, so not much smaller), but other than that they seem a decent size. They’re actually Basil bike crates. Hope to order one next week. Not sure whether to get dusky pink or black. They’re also drilled for racks, so it might be easier to attach it to the Clydesdale fork.
Edit: FWIW the other crate I used was W56.5cm x L38cm x D31.2cm (internal) and 66L