Why not screw some plywood to the bottom of the joists to spead out the weight and then secure the drop down mount to the plywood.
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SOB
2015 Volvo VNL780 D13, iShift, 32k ET air hitch, Idle Free LiFePO4 APU
16.8KWhr LiFePO4, 2.52 kw solar, 10kva Victron split phase
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