I can understand why the OP is thinking along these lines. The way hire car firms treat customers these days is little short of a disgrace, seeking to land outrageously inflated repair costs on them for damage that is often more imaginary than real. In this case, I can well imagine the size of the repair bill that will be handed to the OP. It's only if you've paid the extra premium to waive the various damage excesses on the policy that you'll get away without paying anything, and the fact that the damage may have been caused by the criminal act of a 3rd party makes no difference to your liability. (Yes, the hire company will have their own insurance, but why would they claim on that when they can land the cost on you? Your hire car policy will make you liable for any damage caused to the car during the hire period, irrespective of whether it's your fault).
Having said that, in this case trying to fix the damage yourself is not a sensible course of action, and may well ending up costing you more than your liability under the policy. You would certainly be breaking the terms of your hire car policy, thereby invalidating it, with the result that they could land you with the entire cost of any loss they experience, including any consequential loss. It's a bitter pill to swallow, but you're better off coming clean to them and taking it on the chin. I can certainly recommend the insurance policies that are widely available online and that cover you for any damage excess, and do so far more cheaply than the hire car companies (I've got a 12-month policy that covers me when I hire a car in the UK or overseas - check out the discussion threads on moneysavingexpert.com), but unfortunately that won 't help you on this occasion as they don't work retrospectively.