I almost referred to this as a Sidecar. Can that term work? It's definitely sexier than "module"

I've decided on a whim to mount a second trackpad to this thing.

So here's the idea.. trackpad slides into this unit, and the screw tension holding this on is enough to hold the trackpad in place. No, not quite...

OMG, I just got a GREAT idea...
The Trackpad slots in, the module is attached to the main chassis...

and then this orange decorative piece is screwed in, and it's actually a lock block, putting tension on the aluminum base of the trackpad!
Pretty clever, right?

Final touch is to add the mounting holes to the back...

Looks pretty good!

... and since I can't leave well enough alone, let's add some decorative flair that is also a support structure...

That should do! I bet it can print fully without supports. Though the design isn't my favorite.