The spools should spin as you pull off the thread you want, the holes at the ends of each bar are to tie them together. It should (in theory) stand alone or stack neatly.
My thread spools are the little card ones, 56mm long and 11mm wide, if yours are different you might need to tweak it so they fit.
I might make some from wood with skewers for pins.
Top/Bottom: 165mm X 10mm X 10mm.
Slots: 15mm centre to centre and centre to end, 5mm deep and however wide your pin is.
String holes: 5mm from edge, 2mm between edges of holes and sides.