In order to allow portals to remain on moving surfaces set the "sv_allow_mobile_portals" console variable to 1. Use a point_servercommand to do this. Be sure to set it back to zero when you no longer need portals to remain on moving surfaces.
Portals will display the opening animation a second time if the surface ever stops moving
Note:Portals on moving surfaces behave differently than static portals. They calculate physics much more loosely and can fling passing objects around at strange speeds and angles.
Note:Placing 2 portals which will collide upon at least one of them moving, if you stand right between them just right, upon the portals' collision, you will be teleported into the void freefalling forever in darkness. As of yet, nobody knows at what point in the void you are transported to or how this is determined.
Portals casted upon a moving surface