Sorry for your lost. But it is only material and nothing that cannot be replaced.
Not sure in CA and your policy but I keep all my mod and tool receipts and picture files in a fire proof safe and make sure my policy covers such things. I would not take no for an answer easily with any insurance company, make them pay, you have paid them plenty I am sure!
To get the mods they would have to be claimed on the insurance. Usually not many people do that as it drives the insurance stupid high.
[MENTION=322]CanadianST[/MENTION] That said if you paid via a credit card call the credit card and you can have some of the money replaced depending on how long ago it was purchased. I did that for theft once. They may ask for proof of destruction but that would be very easier if the police and fire department are involved.
Thirdly the storage should have content insurance as well may want to look into that as they would reimburse a portion of the investment as well.
4th ly(?) you can also talk to your insurance about content insurance. Part of your insurance should be replacing "things"
in your car, usually cash, jewelry, laptop, etc... You know the stuff that is not part of the car. If you had anything like that in the car you should get a return, I know mine is capped @ $1000 but it could help for things like floor mats (if you had expensive weathertech cargo or other), etc... (I would not try to claim the mods that are bolted in). I know it sounds petty but it is your call, you already have a claim adding to it does not affect the premium more. Just make sure you tell them it is the SAME claim and you forgot to include content.
Good luck.