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Unsure if it helps your situation but we use an ex MOD mess box (aluminium and lockable with handles) on our Omnium (it is conveniently the exact width of the front rack) which we have used zinc P clips with the nuts on the inside of the box meaning it's easy to take the box off if you have a key but if you don't you can't.
I'll post a photo soon.
I'm not sure it'd quite be big enough for what you're after but it's a sturdy, lightweight and cheap (£40ish) solution for a cargo box
We've had customers who want custom racks/boxes for Omniums in the past and just used local fabricators, not hugely expensive if you're after something simple but specific.
Vince
M_V
VeloSurMer
Does the shampers need a resting period after being carried on a bike? I imagine that it'd get a bit bounced around no?