On the left side of this picture is an unmodified 1" x 1" x 1" PVC pipe tee.  On the right side of the picture is the same tee shown after cutting off most of the tees arms so as to leave only a thin ring of plastic to encircle the tube.  The lower part of the tee arms has been left in place so as to form a saddle to securely support the tube.  A support such as this does not block much of the light from the tube.  The support is also non-metallic, so it does not interfere with the RF signal in any detrimental way.  The tube may be safely fitted into the mount by gently filing the inside of the support ring with a round "rat-tail" file just enough to allow the tube to be a slip fit through the supports. The tube may then be anchored safely in place by applying some clear "RTV" adhesive to the tube where it goes through the support ring.  DO THIS ON ONE SUPPORT ONLY!  This will hold the tube in place securely, but will still allow the tube to move slightly  in the uncemented mount as the tube expands and contracts during normal operation.