(1) Tell us what you've considered already, and why those ideas seemed workable or not.
(2) Very few of us know your car well, so help us out with more information so we can come up with ideas: Where does the OEM nav unit get installed? How big is the Pioneer unit? What in the dash are you willing to sacrifice?
For instance, what are the two big slots at the top of the dash? Could they be replaced by the Pioneer in a housing? If you went on top of the dash like that, you could build a panel and housing to fit there. If molding a system like that is too far beyond your skillset, you could hire Nexson to build the panel once you tell him what you need. You handle the mechanical installation, and he'll make it look OEM.