I designed this for my quad line kite.
The lines can be hooked over the 8 small pillars at start of winding.
It has holes in the corner for the elastic to hold the line in place. After winding, the elastic can be pulled over the corner to the front to hold the line in place. See second picture. Because the elastic is hold in place on both sides, it is not in the way during winding and unwinding.
Added the freecad design file.
In order to fit my printbed , I had to rotate it diagonal over the printbed, and make it slightly smaller.
Possible use a high fill to make it strong.
No support needed.