At a small station such as FSD and if you live close enough, why not take a spin out to the airport and speak with the manager (will need to do this during regular business hours) and explain what you are looking for and see what can be done. Are you prepared to sign a contract obligating you to the # of days you say you will need the vehicle? That may go a long way.

On the insurance front, check with your new employer, but if all of the rentals are work-related, it may be that the employer's insurance covers whatever there is and all you need is an umbrella policy.