Does your sales invoice from the shop detail the stove, flue, and the parts required to fit the cleaning plate or box? Were all parts specified delivered? If something was listed that wasn't delivered, then you have recourse with the shop, but that really should have been addressed at the time.
The details around fitting, and the inclusion of a cleaning plate or box is a matter between yourself and the fitter. If you paid for parts that the fitter collected but didn't fit, then he has your property and should return it. But again, that's between you and the fitter to work out.
Not really an ideal solution to this from your point of view, but perhaps a better fitter might be able to retro-fit a solution.