Looking again at the photos, i would guess go for a top deck, as i think you will get bending of the chassis ? if you mounted it on four screwed posts, you could maybe have all your electrics on the deck and have it as a battery strap, also this may help a bit if you are getting any twisting of the chassis.
What does the chassis feel like at the moment when you try to flex it ?.
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Jonathan | Atomic-Carbon
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