I’m sorry about your injury and losing faith in diy.
I’d venture a guess the battery was just sitting in the enclosure, right? the weight of the battery probably was too heavy for the enclosure mounting holes to carry.
to mitigate this many of us are actually binding the parts to the deck first using velcro or something of its kind. then just using the enclosure to protect the parts as a cowl of sorts.
$1000+ shipping is a great price for the complete set, I bet someone will jump on it quickly.
maybe another option would be to have one of the more experienced builders, preferably close to DC area, to put it together for you and try one more time?