Exterior Rear Door Storage solutions - any ideas?

[Sparkyspice]

I have a L3 H2 17 seater minibus and I need more storage (for size, think Stanley FatMax XL toolbox or Ikea 50L Samla box) and it needs to carry about 30kg of kit. The obvious solution is to mount something on the towbar, but the rear door(s) must be an emergency exit, so that can't happen.

I thought about a Fiamma bike rack, or a ladder attached to the offside rear door and then attaching the storage box to that.

Has anyone got any other external solutions (that don't include a roof rack or roof box!)?

This minibus is used on a restricted PSV licence, so I have to be very careful not to obscure the numberplate, or rear lights. Also the bus has rear windows, which makes mounting things a bit trickier...

[Altransit]

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