Mike,
You will need to know what adaptor is on your Aptus, like Hasselblad V or 645 or Hasselblad H, i am not sure how many there are. Then you can look for the appropriate sliding back, etc.
Kapture Group makes sliding backs for a lot of cameras. Also Digital Transitions and Capture Intergrations(might have missed the spelling) are digital back dealers and could easily have used sliding backs that could work.
You could also troll ebay for a sliding back for your camera, but that could be a very long wait for one to turn up.
There is one other possibility called a Graflok Universal 4x5 back, your back plate and ground glass 'pops' off and is replaced by this, but you would need to purchase an adaptor plate to go on this graflok back that allows your aptus back to mate to it. The basic problem is you can have the groundglass on your camera or the aptus and it quickly becomes a pain to switch them.
Also remember that your 4x5 lens become a lot longer because the sensor area of the aptus is about 1/2 of a 4x5 groundglass.
Leaf makes great backs, enjoy.
Brian