Hi Nico. Since seeing this post I was also very interested in going to Duiwelskloof, and we did (twice). Perhaps I can give some advice here, as we learnt quite a lot. I am stricken between giving information and not saying a thing, as it is quite a dangerous location so should not be taken lightly.
Access is through the Banhoek conservancy security in Kylemore. There is security 24/7, so cars are safe. You pay R50pp/day via Zapper, before the time or on the day (there should be a QR code at security). No one checks whether you paid, but it is a good gesture as the place is amazing.
Getting into Duiwelskloof, the first time we went up to the “Sky Traverse” and then walked across. This took us a whole extra hour and was a bit detour. The second time, we entered the conservancy gate, went left, and kept as low as possible for as long as possible (skirt the farm fence, then aim for the oak trees, after the oak trees, follow jeep track). Eventually, you will reach a MTB trail called “Digbos” - this is probably the easiest path to take (and then follow your nose to the entrance of the kloof). I can send you a picture of the map notes I took on Avenza maps. Send me an inbox and I can whatsapp it to you from my phone. On the walk in (and out), look out for Eland and Gemsbok. We saw the big herd of Eland on both occasions (20 plus strong).
Navigation - the conservancy uses the app “Trailforks”. It has all the trails on it. But it is not very user friendly. If anything, it should be used to find the trailhead for the point where the jeep track links up with the “Digbos” trail. I used Avenza maps for the topographical layout. Another obvious thing - once in the kloof there is no GPS functionality but the topographical map helps a lot with navigation.
In the kloof - this kloof is very very wild. Expect a mix of offtrail forest walking, big loose scree slopes and lots of uphill. There are a good number of cairns so finding your way is doable (if you are savvy). I think the whole kloof is about 6-8km, with 1100m of climbing (no good path). We went twice and never made it to the very top (heavy packs, lots of chilling). There arent that many pools, so if you see one you should swim.
Campsites - we found campsites at the first 500m of the kloof the first time and the second time we slept just after the scramble (you will know when you are at the scramble - its pretty obvious that the only way is up). On this second occasion, we however experienced severe baboon-induced rockfalls from the 600m high cliffs above. Had to evacuate our camp 30min before dark but luckily found a safe spot to sleep. So, this is very very important: consider rockfalls when choosing a spot to sleep!!! We had a legitimate near-death experience, the clostest rock fell 2m from me. There isnt much real estate in the kloof, you will have to get creative. It may be easier to find spots to sleep without worrying about a tent (I did pitch my small tent once). I know that there is an “overhang close to the top” which has been used as a sleeping spot and sounds safer than the spot we chose. But then you have to make it all the way up! Please just really understand that this is not a place to take lightly, it is closer to “canyoneering” than hiking. I would say you should either aim to set up camp early, walk up and back down to camp; or you should aim to reach the “overhang near the top” (this is what I would do next time).
As I have said, I havent been all the way up but I could imagine it would be quite the outing, especially with overnight packs. But is definitely doable, just start early. Hope I didnt scare you, but the rockfalls we experienced was a major eye-opener for me…