Difficult to say without seeing your idea put together.  If the perspex subframe has a sufficient air gap from the tube itself and is made of thick perspex capable of carrying the weight of the tube, then it should be okay.  If you are asking the perspex to span a considerable space without good support, I would advise against it.  The last thing you want is the perspex breaking, the tube moving and potentially damaging it and other parts of the machine.