This is just a suggestion but a very good place would be zambesi mall on the corner of the old zambesi and moloto roads, its pretty much deserted and there isnt much going on inside the mall or the multistory carpark.
Its a good place at night because of action sports there meaning the carpark is open at night.
If anyone has a question about the place just ask and I will respond as i can.
Cheers and I hope that dogging becomes a reality in this country