Road permits (Gaborone)

Good day,
I am currently driving with my South African number plate in Botswana. I heard that the temporary road permit is only valid for 90 days etc. I have however heard through the grapevine that some people are able to have their foreign registered cars for a longer period (2 years?) in Botswana without having to pay anything at the border upon each entry (as I will be driving to South Africa frequently). I heard that they purchased some form of annual permit. Does anybody have more information regarding this? Where can I get such permit? I would prefer keeping my SA number plate so that I don’t have to re-register my car once I move back to South Africa again.
Any advice?

Thanks,
Conrad