Cringe-tastic, but still, I'll tell you there's a few problems straight away. If you go for a bog standard iPad, they are 10 inches. So you've got a problem right from the word go. Double DIN is 7 inches. What most people have done when it comes to cartablets, is taken a 7 inch tablet, something like a Galaxy Tab. I guess the iPad Mini would work for this too. Your other issue is that sound from tablets is pretty poor. Most people who have made these kind of projects work, have commented the sound quality from a Galaxy Tab or iPad or whatever is really naff compared to a proper head unit. That said, if you go for an iPad Mini, it's easier to simply make a mould out of an FP-17-00 I imagine. If you're an Apple fan, then I really can't help you, but if you are willing to go for something Android'y, I've been eyeing up the possibility of doing a totally unique head unit that features Android Jelly Bean, plus could be upgraded very easily every few years, quite cheap as well. I've not seen another one like it, but I've not completely convinced myself it can be done yet.