Auto Belay alternative

Does anyone know of a cheaper or alternative system to TruBlue Auto Belays? I'm planning on completing my ultimate home wall (it's actually in a barn) and there is big vertical space (approx 12x16 feet) that I'm going to make into a cruiser/ARC wall for the kids. I had it all planned out and then thought about how cool an autobelay would be for long circuits. Then I priced them....cheapest I could find was around $2000. So...no dice!

It only needs to have 16 feet of tether, so even if I had the cash, the shortest AutoBelay I saw was 24 feet. Other than self-belay practice or top rope anchors, what is there that could possibly keep the dream of hours of uninterrupted, mindless endurance laps alive? 

kid-rope to pulley-rope from pulley to another pulley a few feet away- rope from pulley-counterweight that is less than the climbers weight
Crude version
Cute version

Or REALLY good insurance.

The kid counterweight pulley thing is awesome. Going to mess around with that and see how it works.

There are a several reasons why mattress stack/tons of padding won't work. Firstly, it's in a barn. Like a proper old tobacco barn, not a big metal shed designed for climbing. There are cross beams every 4 feet or so. So even if you had the highball pad stack to end all stacks, you are very likely to wreck yourself on a 4x4 before you get there. Secondly, I'm 39 years old. 16 foot falls hurt no matter how much padding is down there. Thirdly, it's primarily for my kids. They are crazy, but having my 6 year old Tarzan swan diving out of the rafters from 16 feet gives me the vision of many extra trips to the emergency room. 

(removed) due to tricky and dangerous lowering sequence the proposed method is not to be used by kids unattended.

Edelrid Ohm. Believe me. Just joking, but it really does "auto belay" it's just not a recommended use, obviously.

How about this one? Looks like it is in working order, just no longer supported.
​https://www.geartrade.com/item/546661/dynescape-auto-belay-cheap-for-parts​​​