Another option is www.insurance4carhire.com if you don't want to sign up for another card. This is a UK insurance policy but maybe non UK residents can apply also?

I had this policy for a year but now have the Amex Platinum Chargecard (note car insurance is NOT included with the credit card, just chargecard). If you have a small incident then the rental company charge the repair to your card and you claim it back from Amex. If it is a larger claim, such as a write off, theft etc then you submit a claim to Amex and they liase with the rental company and no charges will be applied to your card.