That is such an outstanding mirror. I'd fiercely endeavor to protect it from agents that could harm it. I like the sturdy Clydesdale case. You're on the right track. I have some ideas to consider.
I'd be concerned about outgassing from the padding material that may dull the mirror coating. Also, I'd take care not to leave trapped moisture inside the case which could promote the growth of mold and such. Some moisture from a dewed mirror could find its way into the pores of the padding. Proper ventilation along with dust control is the goal.
The acid free tissue is only appropriate for a scrupulously clean mirror. Dirt that accumulates as the mirror is used, would act as an abrasive when the mirror jiggles inside case during transport as Kunama has suggested. Bottom line - nothing should contact the face of a mirror that has been in use out in the field.
Some time ago, I read about a mirror box design where the mirror restraints were angle-shaped so that they contact only the bevel. There were four restraints, one at each corner of the box. Kunama's dish concept is good because it leaves room above the mirror so the coating remains unmolested.
I think that your mirror should be restrained circumferentially from below with about six stiff, seasoned wood blocks. The bottom of the assembly could be Baltic birch ply to hold the circumferential blocks firmly in place. The contact surface of each block can be covered with felt to mitigate shocks from impacts. Not much wiggle room needed, just enough to slide the mirror in. Pad the seat where the bottom of the mirror rests appropriately.
Now that you have constrained the mirror from excessive lateral motion, you must restrain it from moving up and down. That's where the inclined plane contact with the bevel comes in. It can be done with a router. Rout from both sides of the Baltic birch plywood sheet. The bottom rout is made with a tapered bit only part of the way through. Use the height of the lower restraint blocks to guide your depth. The top hole is the size of your mirrors coated surface. You end up with only a narrow rim of plywood contacting the mirror.
In order to index top and bottom restraint assemblies together, set three dowels into the wood blocks that come from below. The dowels fit into corresponding holes in the top plywood section to align the two accurately together.
So first, mirror drops in between the six lower block restraints. Then the top plywood crown is set in place, guided by the three dowels. Finally, the case is closed and latched. Hopefully you will have enough padding attached to the top and sides of the case to keep the inner payload from shifting around.
I think this design modification will protect the coating from abrasive jiggling during transport while providing adequate restraint and support in three dimensions.
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