So I did some quick maths and deduced that the tubes alone for the gantry (that moves) is 9kg for the Z axis to move. Making the bed move is not possible as this would fail the belt bed solution. That constant force spring I got a while back is a clock spring and the Aliexpress seller probably just threw keywords in the title to get shown in more search results. 

Voron uses KeyBak for their switchwire, which is a retractable lanyard, but the price to force (and usable length) is pretty poor. All other lanyards also seem to be of low constant force. Retractable dog leashes might be an exception but I doubt it. 

So I'm scrolling pages, feeling defeated, when I see post #3 on this forum page. So I go onto Ebay and he's right: they're cheap and can be find in a range of strengths and lengths. This is the first listing I clicked on, which prices a 400mm 800N spring at £11.99. I'll be looking for an eyelet version.

After confirming that gas springs do exert a constant force, and thinking about this recent Slow Mo Guys Video, I started with some requirements and assumprions:

• Expected axis weight to counterbalance:  12kgf
• Max achievable length: 2.5m
• Max usable extension: 380mm
• 1kgf = 10N

Which means the maths is:

• 800N / 120N = 6.666 
• 0.38m * 6.666 = 2.533m

Oh. How convenient. That worked out rather nicely. 

I feel like I can use some of the excess 608 bearings and M8 bolts to make some pulley. I can't imagine a suitable cable would cost more than £8, so I could potentially have a new solution for under £20, down from the > £63 solution I was originally thinking of with the clock springs. 

Oh but pulleys can only be set up in integer values. Hmm.

• 800N / 7 = 114N
• 114N = 11.62kgf

I'm sure that's close enough.

Obviously, this also means that it should be much more scalable than the constant force spring approach. The SleepCinema only needs around about 0.7m-ish and not 2.5m like if it was a single SecSavr going from the floor to the ceiling. That would only need a 100mm extension so even the smallest (150mm) spring would suffice. I'd still like the option to reconfigure a taller SecSavr without having to buy new components, especially if I'm only saving like £3 per shorter spring.

[E1 - 16:17] The only concern now is if the springs I've seen are damped or not. Next, I just realised/remembered that the springs are described by their max length, not by their stroke length. Lastly, this image makes me think that it's not all that constant, though the forces are not labeled so I don't know.