You could probably ebay a decent enough frame for the box by searching "car trailer" and finding one with nackered electrics/axel. The wheels on most of them unbolt and then you can use them bolt holes to secure it to the cargobike spring bits, might also need to grind off the towbar. Something like these.
Once it's on you can then drill holes and easily bolt on more box section to build up the frame on the sides. For the handlebars use some flagpole wall mounts or railing mounts, might be hard finding something in a bike size(25.4, 26, 31.8) but you could get close then shim for the brake levers to fit.
You could probably ebay a decent enough frame for the box by searching "car trailer" and finding one with nackered electrics/axel. The wheels on most of them unbolt and then you can use them bolt holes to secure it to the cargobike spring bits, might also need to grind off the towbar. Something like these.
http://www.ebay.co.uk/itm/car-trailer-/281589531660?pt=LH_DefaultDomain_3&hash=item41900b440c
http://www.ebay.co.uk/itm/Car-Trailer-Half-Ton-Units-/121562963497?pt=LH_DefaultDomain_3&hash=item1c4db79e29
Once it's on you can then drill holes and easily bolt on more box section to build up the frame on the sides. For the handlebars use some flagpole wall mounts or railing mounts, might be hard finding something in a bike size(25.4, 26, 31.8) but you could get close then shim for the brake levers to fit.