1/4 of an inch has worked for me. I'm using turnbuckles and "U" loops with nuts on the other side of the 3/4 plywood board so mine doesnt move when driving. Haven't had any problems with wood on plastic rubbing.
The real MVP right here lol. Looks like all the links/photos on the first page have been shut down, would be nice if MStudt could reupload maybe on imgur or something :thumb:
Edit: reuploaded this image to imgur just in case the above goes down
http://i.imgur.com/uMa1uwq.jpg
