Yes, I love magnets too. I use them in most of my designs even ones that do not have connecting parts. 
The thumbscrews would of course be optional and there is the additional negative factor that thumbscrews of the size I would use, which would be Computer PC Case 6#-32, one would have to re-tap the thread holes. But I believe that the option is necessary to ensure rattle-free mounting.
Given the amount of vibration, even rare earth magnets would succumb to failure not to mention the high degree of likelihood of vibrational noise. So whereas one could put very powerful rare earth magnets to counter this, they would have to be strong enough to counter the vibration. There is the added high cost of such magnets.
Here are two examples of magnets I have had a high degree of success using in my prints.
For massive holding power such as a handle grip, I use these. While at $3.75 a piece, they provide holding power like nothing else I have used.
Then for longer grips where space is a consideration, these are what I use and what I would consider using for a magnetically mounted plate. Again, depending on how many one might use, the cost can add up.
I’ve used both of these in my designs depending on who has the better price that day. I can say this, anything less than 5mm of thickness will not work well if one goes beyond 1mm wall width.
I’ve tested these 3mm and they only work well if you have a mating magnet on the other piece. If you are using them against metal, then you have to make the wall thickness 0.5mm which really creates a weak part.