This is my entry for Joel's Spool Holder competition. I used the 18 mm measurement for the board thickness. Instead of putting the spool sideways on the board I extended it out front so the other spools can be accessed easier. The spool holder is broken up into two parts for strength and is put together with a snug fit. If the fit is too tight just blast part of the arm that the bar slides into with the heat gun to make is a little more loose - I know Joel has been having fun with the heat gun recently so he may like that. Hope it works out!