it's semi-custom. it's the factory nav housing with some modification. when i originally installed my carputer, the mp3car g35 mount didn't exist. i bought everything in the below picture minus the display and it was a very tight fit but after a lot of cutting i got my now 4 year old xenarc 700tsv to fit into it. these parts cost me a good deal more than what a mp3car g35 mount would have but at the time i had no experience fabricating and painting so it was my best option.
this is what it looks like installed.
because the mp3car transflective lcd is smaller than my old xenarc, i was able to get it to fit inside this housing and it fits after switching out the lilliput controller board with the xenarc's (because of the 800x480 issue). this info probably doesn't help you since i assume you have the mp3car g35 housing.
this is what i know about the size on the transflective compared to xenarcs. i have a 4, 3, and 2 year old xenarc. the newer xenarc lcd panels and the my transflective lilliput lcd panel are the same size. mine were both hitachi panels. the 4 year old xenarc lcd panel was a little wider. xenarc controller boards are taller than lilliputs and the inverter is a seperate board while the lilliput controller has the inverter included but the buttons are a separate board. i would guess that if an older xenarc fit into the mp3car g35 mount, then a mp3car transflective would fit as well but you may have to lose the buttons. if the ribbon cable between the button panel and the main controller board is long enough, you could relocate the buttons or extend the ir receiver. i hope that helps some.