Ian, this is likely to be an iterative process with several printed protypes required before a satisfactory item is produced. Best would be to let the designer have the old cover but, clearly, there is a risk in that if postal services are involved but other wise as detailed a sketch as can be managed, including various radii for the corners etc. Details of the base to which the cover is attached would also be useful to get the various clips correct.
Where do you live? Also, in what material would the final cover be produced.
It’s the sort of thing I’d be happy to have a go at.
Good luck with your quest.
Rod