sounds like under the seat is still a good place for you. You can get cases that fit the min-ITX MB's. VoomPC has a case designed for it. I doubt that under your seat will see enough amounts of water to affect your computer. On the other hand if you're thinking to switch between dummer and winter, maybe a laptop would suit you better but that doesn't solve the problem of screens. You'd either have to get two screens or make your accessible enough to swap between cars. Make sure you read through the FAQ and take a browse through the Project forum. You'll get a great understanding of what is involved and a lot of examples of people who have done it before you. Good luck!