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This is exactly what I’d visualised last night!
I have some spare spring bar things from cheap shoe trees, they are pretty solid, but maybe more difficult to mount. Will have to hunt for bed springs now!
@MCamb no idea on max load. The oldboy who made it was 11 stone and happily stood on the platform, and normally makes these with a wooden kiddy box like a bakfiets to transport kids in.
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You can get steering "stabilizers" ("stabilisers" often wont show them in a search) online if you want to pay for a more basic one than the bullit one but idk if they'd work with that tube size/shape. I think they're intended more for dutch bikes more than cargo bikes.
hollandbikeshop.com have a few
Hole in the fork crown, bed spring, jubilee clip around the frame just past the bend between headtube and wooden bit.