A suggestion...

If you are indeed going to be on a cart, might I suggest covering it in chalk board paint?  Some of the time you are in another teacher's room they are funny about you using "their" chalk board.  It is really not a problem at all to make the sides of your cart a chalkboard with all of the paints available.  But I must suggest the green ping-pong paint.  It lasts long and is not a harsh as black.   

Something I will be toying with this year when I make my two new carts is a MAGNETIC chalk board.  (Feel free to OOOHHHH and AAAAAHHH)  I may even try a magnetic WHITE BOARD, as they have that kind of paint now too.  I know, I know, I'm a real risk taker.  I'll have to let you all now how that goes...I haven't had the nerve to even do a test.  BOK, BOK, BAGOOCK (As my imaginary chicken would say!)