My reasoning for putting the aluminum inside the platform and clamping it down to a flat surface while the epoxy cures is that you can now adjust the side arms which may not be perfect.
When you clamp it flat, the arms may move a little because they were not 100% straight before, due to the warp in the platform.You can now adjust them without flexing the platform.
Putting the aluminum inside the platform allows you to use the exact same settings as the B9 came with as there is no change in the z height.
Sanding the aluminum after doing this makes the resin bond really well to the aluminum.
Glass can be used also, but gluing the glass on the out side may require some z height adjustments if your items loose some thickness due to the extra thickness of the glass.Basically whatever works to get the platform dead flat will be beneficial.
Buying the new stainless platform is the best solution, but if you are overseas, that can get costly.