here's a thought?
the 'blue snap line' chalk use to come in cones or cakes, but i believe it was white back in the day. the user had to pull against the chalk to get enough chalk on the string to do the job. i believe i remember [believe me there is some question!

] seeing these cakes of blue chalk, or i guess white chalk, with string markings through them. i asked an old guy, "what it was"? he said, "speak in complete sentences"! actually he said 'who wants to go home with blue fingers?' naw just kidding he said if you get home first put a chalk mark on the board, and if i get there first i'll erase it'--this part is true...funny what you remember and less funny what you disremember!!!
there appears to be string markings on the wood thingy. the string had to be pulled tight. the question is how did they get the curve in the wood?

ha ha! maybe a recycle part of a chair brace or some other ingenious device home made better than store bought!
also, the width of the straight piece looks "handily" handy?
hope you don't mind the 'chatter' in my finger tips! just pitching in!
this sight is fantastic with all the people giving honestly, freely and intelligently. just goes to show when ya gang up on a problem ya might just fix it!
mike