Junior had one so I had to self-draft one for Little Lady too. The first version is pretty simple and will be followed by a second one made of some nicer fabric.
Sizes are shown on the pic (fits 2-4ys, for older kids the length needs to be increased only).
The cool thing about it that it can be adjusted by pulling the straps on the side and no need to make head measurements.
The tubes to guide the strap is made using a 5cm wide bias tape (mine was 33cm, but it really depends on the curve) attached to the raw side of the curve. The strap is 170cm long (1-piece).