I've seen a sled deck tied onto a flatbed before. It caught my attention but it must have worked. I'm thinking of a flatbed on my pickup but one thing I'm wondering about is: I would thing that road slime would still blow around the sides and get all over the sled. That is one thing about a deck is it puts the sled up high enough and with the extra width it blocks nearly all the road grime.
You can get a toyup deck for cheaper than a flatbed costs and its a great deck too.