The shuttles had a mounting platform type of thing that would be installed in the back of the payload bay to hold hubble in place while it was being worked on.

The shuttles had a mounting platform type of thing that would be installed in the back of the payload bay to hold hubble in place while it was being worked on. This also freed up the robot arm for other uses. This mount is probably sitting around in a warehouse in Florida somewhere doing nothing. They should dust it off and make it available for these proposed future missions.