I'd be inclined to use 2 belts to prevent the "Y" from skewing... Have a look on fleabay for Carolbrent aka HubbardCNC as that seller lists open ended belts and suitable pulleys (not available to UK buyers...Grrr
) Also consider making a reduction drive using belts/pulleys for as backlash free an installation as possible (on the cheap) Oh and check out the linear slides with skate bearings they sell? hth