aren't there third party insurers that charge decent rates? Like when u buy travel insurance?
There is a company called Allianz global assistance (Formerly Access America), that sells collision insurance for rental cars. I'm not sure if they sell liability as well.
1866-807-3982
EDIT: I just spoke to them. They charge $9 per day up to 45 days. This is for collision only and covers as the primary insurance.