My opinion, time/labor/money wise it would probably be better getting something already designed for an X. Don't get me wrong, I'd personally be the one rigging something I could get on the cheap on instead of shelling out the required coin so I don't condemn the idea of it, just remember you need internal bracing as well!
As for pro/con from someone who went from internal to external, my list:
More room in the cargo area +
Blocked window/blind spot area reduced +
Less of my stickers easily viewable -
The added area needed to open the carrier to open the hatch / I can see it as a burden to some but I rairly have to parallel park-if I do with plans of needing to open the rear I leave room and I just pull into parking spots at stores instead I backing in
More external mounting areas for stuff +
More things to grease + to me, not so much to most
Really it's person to person but I consider it a positive I did switch (though I do think sometimes He looked cute and clean without it)
You could always fashion a roof mount setup as an option as well, but climbing up there could suck as well... (Get AAA and make them do it! )