I just used a quality car seat strapped in tightly with the stock center seat belt. Make sure you have the car seat straps adjusted properly. A top, hard or soft, is a good idea to keep branches from hitting the little ones.
A funny story:
When my oldest was about 4 she went to daycare for about 3 hrs a day during the week. Each Monday they were asked what they did over the weekend. My daughter says "We slept under daddy's truck. Then we drove in the woods, and tree hit the window. The glass exploded but I didn't get hurt because it was safety glass. Then we had a picnic on top of a mountain" Well, the teacher tells me Jessica is making up stories and then tells me what she said. You should have seen her face when I said "That's pretty much how it happened".