Firstly you're being a wimp by not putting it at the front, always like to start my posts off with a few words of encouragement, but if you have decided to wimp out you have multiple alternatives.
Mount it in a somewhat vertical plane somewhere just behind the roll bar, remember you also need to be able to mount the header tank so that the water level in it is above the height of the radiator. If you mount that on the passenger side then you won't get scolded, as that is likely to be the only place water would gush out, any leaks in the radiator will only be minor initially. It will be well in the airflow, but will still have fans to assist, as you don't have the legally required interior mirror rear vision will not be an issue.
Mount it in the plane of the sloping tubes going to the back of the buggy to hold the subframe directly above the engine, it will still get airflow although restricted but fans will still pull air through, it might not look quite as intrusive, it will still block your vision that you don't have, but you will probably have to remove it each time you want to do anything on the engine & you will lose the option of luggage space.
Mount it at the rear at the same height as the engine, it won't be in the airflow, it will certainly need assistance of the fans even with normal running, it will be on the 'cold' side of the engine away from the manifold, you maybe able to scoop some air down to it.
Mount 1 radiator either side of the of the buggy F1 style & build them into pods, they will be in the airflow & can be fan assisted when required, you could even duct them out the back in a blown diffuser but not sure of the legality of that now with Bernie.
Mount 1 radiator either side of the buggy at either end of the engine, you will have to build scoops if you want them to be in the airflow, but with fans pulling the air in they should be okay
Failing all of that MTFU & work out how to put it at the front.
Alan...