Tom Thiel,
Are you stating you would mount the crossover board up right on edge? If that is correct this would expose the circuit board direct in line to the most significant mechanical impact from any open back driver in the cabinet. Visual would be a basketball hitting the back board. Damping would reduce but will also increase the resonace retention time of any aquired energy which can become part of the orginal intended signal. My methods are much different 1 of which allows for external adjustemt of the resonance point of the internally loaded circuit board. Resonace will be drained thru the bottom mounting method and coupled internally to cabinet bottom and externally to the mass of the floor. This feature benefit can also be done on a preferred external mounting method. I think we have discussed these methods in the past. I know your project is to be simple and of a high value. You may want to offer up these significant options for users who want to max out resonance control. There is also an option to externally resonance tune the woofer to the backside of the cabinet. This entails a brass connecting rod and a special tuning bolt turned by hand .Like you would tune a stand up bass. Significant audible upgrade. I know your trying to increase open surface area to benefit heat dissapation..Resonance tuning also reduces thermal runaway as the heat has a method to transfer as does resonance.. TomD