Drill a 1/2" hole in the bottom of the T-fitting..
Without gluing, assemble all the components.
align the vertical axis centered on the platform.
Cut four 8mm dowels long enough to extend from the exact center point of the platform plane out to the holes drilled into pvc 2 and 4.
Place rollerblade wheels onto the dowels.
As you see in the picture you are truing the alignment so the wheels naturally roll around the axis