I built one tonight out of junk around the garage that seems to work OK.
The pictures are attached.

The way it works is to put your knee on the 2x6 to get the weight off the cir clip in order to remove it. then take the washers off. hold the ramp and let up on the board slowly until the pulley spins off the springs energy.

To put it back together,
Engage the spring into the correct holes in the anchor.
Put the ramp on the splines about 1/4 inch.
Hold the ramp down with the board but not push down on it.
Wind the movable pulley counter clockwise until the ramp is clear of the ramp slides.
Push down on the board to expose the cir clip groove.
Install the washers and snap ring.