Being in Centurion, i feel your pain. Planet Fitness USED to have Squash courts , but they removed them in favor of a "female only" area in the gym.
I've recently played squash at Irene Country Club
http://irenecc.co.za/squash-fees . You are suppose to join with a yearly fee (can be a social member without doing the leagues) which seems pretty expensive but it IS cheaper than gym by far. You can then book courts on a per play basis. So far we've gotten away without being full members (they don't seem to check) , but paying R15-R30 per hour. I suppose they start checking when there's too many people playing at the same time, but when we were there, it was empty and we just walked in there. ...not sure when the league starts, i only play casually too. You can probably get away with only 1 person joining and the other person getting sponsored

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Another option is something like Harlequins in Pretoria.
http://www.northerns.co.za/viewclub.php?club=9 . I've done martial arts at the same place, haven't played squash there, but in terms of driving and distance, it's relatively quick from Centurion (assuming you stay in the Lyttelton/Doringkloof/Waterkloof/Monument Park areas.
I believe there is a club in Rooihuiskraal too here:
http://www.uitsigsquashclub.co.za/ . I don't get in that area of Centurion, so no idea how it is , all i know, i don't want to drive there after work from central/north Centurion (it's a big traffic F'up with silly bottlenecks).
There's Virgin Actives in Pretoria with Squash courts last i checked, Groenkloof and Menlyn, but the people i know that use it, have to book like 2 weeks+ in advance etc. Groenkloof is located well coming from Centurion (again assuming you're in the northern/central centurion parts) , Menlyn , can be a bit of a schlep based on time and highway traffic.
The Waterkloof Airforce base also got squash courts/gym , but i'm not sure which kind of connections you need to join up

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