Yup there are no rules in the AAFTA handbook that talk about lane spacing. As you know, over the years, there has been everything from simple signs that are inverted V-shaped metal ones, to a stake in the ground. The problem without defining the shooting area/line, is that you have the liberty to move around. Rock in your way, move over. Pebble under your butt, move over. Grass in the way, move over. Another thing is safety. slide back a little and that could put others at risk.
Best practice is defined left and right vertical markers. Without ambiguity, there is no room for bending rules. At our range, I have occasionally made lanes up to 6' wide to work around obstacles such as cactus. But no less than 3' wide.
Here are some comments about my thoughts on lane markers...
The Best Height is ~30"+ tall. This allows clear visibility to place lane and positional signs. You don't want your positional indicators to be hidden. Lane signs with this height also allow your spools to be hung from them so they're not in the dirt or mud.
There is a company I found called
Formufit.com , they sell colored pipes. They are a bit pricy, but we used them for the Worlds, and I absolutely love their quality and rich color. We ordered the 60" lengths and cut them in two. We slid them over 2' lengths of 3/8" rebar hammered into the ground. As for the signs, I recommend printing on 8.5 x 11 cardstock cut in half, and using a 6x9" 5 mil laminate pouch. The smaller size is rigid enough to handle a bit of wind and is easier to handle. Not too big, not too small. (Message me if you want the USA-size template I used for Worlds.) Then these are clamped to the pipe using 1" clips screwed to the pipes to secure the top and bottom of the sign marker. This setup will last for years and is top notch.