I have no waterproof case recommendations, though having a specially made fiberglass case specially designed for fitment to my Morgan by Kevin it`s restorer, there are a couple of other considerations that might be worth consideration..?
Rearward view via the internal mirror seems to have played a part in the design of my Morgan case as did a degree of anti-theft should the Morgan and case be left unattended while touring.
The anti-theft part being that the case could be locked shut and the case could be fixed to the rack using "P" clips, the clips being fixed through the base of the case with stainless steel set screws, nuts, spring and penny washers.
Rearward vision is obviously reduced to some degree though to overcome that required the ability to adjust the positioning of the internal rear view mirror seems key. Internal mirror adjustment on my Morgan was no problem, it being fitted with the type of mirror that can slide up and down on a supporting post mounted between the top of the dash and the top of the windscreen frame... With the case fitted the mirror required to be raised up the post to provide the most advantageous angle out of the more restricted view, more important to get that right when the hood is fitted.
I had thought to have a couple of soft bags made to fit inside the case leaving it affixed to the Mog rather than get involved in dismounting it at overnight stays, but never got round to it....!
I hope this little rambles assists in your planning. I hoped to post a pic of the set-up but seems I have fouled up that process yet again...

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