Going on a very long car trip this weekend, 28 weeks pregnant?

I will be almost 28 weeks pregnant on saturday when we have to get in the car and drive for 14 hours, then 30 weeks when we drive back. What's the likelihood that I will get DVT or something like that? Also I wanted to buy those pee-mate things so I can pee by the side of the road but now don't have time to order them, how could I make one myself? The last time we made this trip I was 14 weeks and got hugely swollen ankles on the way back home... does this mean it will be worse this time?