Bummer...too late now, but in the future:
1. Always do a full video walk around of the car on the lot, and include the background so its clear where you are...if you can get an employee on the video, even better.
2. I use Amex with premium car protection...which is something like $18 for the full rental. It supposedly covers EVERYTHING and has a zero deductible...not sure how good it is as I've been fortunate never to have a claim, but Amex has been really good with virtually all their "benefits" I've used over the years. I don't travel that much, however, so it's not a big inconvenience...probably only rent cars a few times a year max.
Good luck with your claim!