Hi Ellie!
I have a detolf and I like the sound of this project
A few initial thoughts
-A concertina lid sounds cool but I don't see how it would be more secure. The more sections you have the easier it would be to lift one section? A cat doesn't need to lift the whole thing to get in and cause trouble it just needs to shift one end. I would be more inclined to make the lid one piece. A lot of people split the lid only to make it easier to lift which is the opposite of what you want.
- You may be able to fit standard bolts on each end of the lid, if you make the lid side-pieces wide enough for the main barrel of the bolt and only have the small receiver end screwed into the end piece. This would take some careful measuring to make sure the timber you use for the lid is the right thickness to have the top lid surface and the end pieces flush with eachother.
Something like this might give you a bit more leeway for things not lining up perfectly
https://www.diy.com/departments/smit...1733036_BQ.prd
- The wires are fairly sturdy, more like bars, and you can flex them sideways towards the center of the cage (if you get me) but vertically there isn't much give in them. I don't know how heavy you want this lid (what is your cat's deadlift like haha) but you'd be doing well to make it heavy enough to risk the bars. You will know if it's too heavy when you see the bottom bars flex in the centre. I've had my lid weighed down with a 3kg bag of sand and no problems but I haven't tested it beyond that.
What you definitely want to avoid is supporting a heavy lid on the glass since that won't give you any warning, it'll just shatter
This is so hard to talk about without diagrams!
Just as an aside, is it the DIY cage with the Linnmons you're considering otherwise? I'm actually just in the middle of upgrading my syrian girl out of the detolf and into one of those.