Daniel,
There is a Tee piece Riley fitting RM98 which may do the job. It is a track slide to suit a 16 mm internal track and has a plastic rubbing strip to reduce friction.

As per the image from the photobucket you would need to cut a slot in the boom section.
Most of the recently built Mozzies use the rope loop. It is simple and effective providing you have positioned the outhaul at the desired position prior to applying too much mainsheet tension. Using the rope loop also gets rid of extra fittings and is possibly less prone to binding than the slider.
Regards,
Peter