As the OP stated, the credit card provided insurance has limits on the number of days for the coverage. He was looking for an alternative that would cover rentals up to 60 days.
Sorry, missed that part.
There are many parts to rental car insurance.
liability insurance (damage to others)
collision insurance (damage to rental car)
personal insurance (injuries to driver and passengers in driver's car)
loss of use
decrease in value of the vehicle due to damage sustained
other fees charged by rental car company
I got that info from the two websites below
Of those coverages, only the first two are really important to have, liability and collision (just like in your regular auto insurance).
Per the first article above, it seems that one option for permanent worldwide auto liability coverage might be buying an "umbrella liability insurance" which offers worldwide liability protection. To purchase that you have to talk to your current auto insurance provider and see if they can offer that.
The worldwide car rental collision insurance can be purchased separately, see an example below, but it seems to cost about $9 per day, not cheap by any measure.
Both the umbrella policy and the worldwide car rental collision do offer 6 months policies.
On the plus side, it seems that in this method you get the primary forms of insurance you need when renting a car (i.e., liability and car rental collision) without having to worry about returning the rental car after 31 days or 15 days.
However, you'd still not be totally insured, for example, you'd still not have personal insurance (injuries to driver and passenger's in driver's car) or loss of use protection.
If you want to be even more fully insured, then you'd need to go with the car rental company's policy, if they offer one.
Another option: just keep bringing the car back (a hassle without a doubt).