Do you think there's any risk to the finish long term using the corner magnets like this?
I think it depends upon whether you are generally rough or careful when you use things. I see the cases in various conditions when they come in for repair. Most look brand new, but a few look like they were stored in a tackle box on a speed boat. The finish should be fine if all you are doing is resting the Dynavap on the corner of the FD and lifting the Dynavap back up to use it. The FDs are pretty tough devices in general - about a year ago I accidentally put one in the washing machine with a load of laundry (D'Oh! ) and it continued to work perfectly after I dried it out - but the finish isn't impervious and will undoubtedly show the effects of use and wear over time.
Hope that helps...?
Updated to add: If anyone is interested in a sturdier case, @Monsoon discovered a very affordable camera case that fits the FD almost perfectly and offers some nice additional protection:
The small bump in the case even accommodates the external switch.