I built this one with a maximum folded length requirement too.
Remington 870 Police Mag. 18.5" barrel, no muzzle device
Choate top folder, manufacture's product code #01-01-12
Total folded length, longest dimension, measured from base of pistol grip = 29"
While not the lightest, or coolest, or newest folding stock, it is very heavy steel and can take some banging around in a truck.
That's the only one I had access to, so I hope it helps.