Now after having built it, I would do a couple of things differently a second time around. I may make some minor modifications. More materials and dimensions to follow - I'll just edit this post.
Wood: I used 690x690 PAR pine (planed all round). Pine is easy to work with and relatively inexpensive. Since it's for indoor use only, untreated is fine. I'd have liked to go something closer to 890x890 but it's not readily available in the local hardware stores. A dedicated timber store (perhaps Penny Pinchers?) may have something bigger. The timber comes in lengths of 2.4m or 3m. I bought 2.4m lengths and cut them to size (sizes to follow)
For the bars going across, I found 20mm galvanised threaded rod to be very convenient. Since the depth of the cage is just shy of 1M (my gym is limited in space and I have plenty equipment), I used 1M length threaded rod and a lock-nut + nice wide washer on the one end and ordinary nut + wide washer on the other end. I just superglued the washer to the nut on the other end - so it all comes off in one go by hand to adjust the bars height - nice and easy. The 20mm galvanised threaded rod can easily carry 150kg each side without issues. Obviously I haven't dropped 300kg from a height in there, but I'm willing to bet it would save your life if you dropped while squatting.
The big holes are drilled with a 22mm auger bit on an ordinary drill (13mm chuck). As you can see, it's nicely braced with 45 deg offcuts left over from the 690x690 timber.
I assembled it as per the earlier video I posted - very easy to DIY.
What I would do differently:
- I would make it a bit wider - like just 70mm wider. Right now, while benching, my elbows graze the horizontal safety bars marginally and it's more annoying than anything else, though it lets me gauge form as I workout. I may still make that adjustment.
- I would have gone 890x890 wood if I could have found it (I have subsequently found it but I'm happy with the end result so I won't go redo the whole thing

).
- I would probably have preferred straight steel rods instead of the 20mm galvanised threaded rod mainly because the galvanised rod acts like a bit of a rasp as you pull them out of the holes - I'm not sure what effect that would have on the holes long term, but I doubt it'll be a problem. Steel places will have these, my hardware shop didn't. I may replace the threaded rods but I do like the fact that I can use standard nuts and washers on it.