RE: Power Inverters for a Freezer
If available where you hunt/harvest, you could just put some dry ice in your cooler?
Would be cheaper/easier than "rigging something up".
We have used it and find that it will freeze previously unfrozen meat and keep it that way. Seal up your cooler with duct tape to prevent the gas from escaping and make sure that the dry ice is wrapped in several layers of newspaper etc. so as to prevent it from contacting your meat directly. (It will cause freezer burn if allowed direct contact).
Just my .02 worth....