Separating old 2 x 4 brick with wheel holder
I actually have one of these that comes apart easily (no idea where the internal tubes are).



It appears that the glue that used to hold them together gave out. You can still see where it used to be, around the edges of the clear piece, and at the tips of the 4 prongs on the clear piece.





If I were going to try to take one apart, I would probably try cutting the glue around the edge with a razor blade, and hope that the glue further in isn't strong enough to prevent pushing it out with a paperclip of something thought the axle holes.