Here is a link to a solid design
http://www.autobody101.com/rotisserie.php
You can do a google search on auto rotisserie plans and/or run a search in this forum. There's tons of stuff out there. If your planning to use it once, it may not be worth the expense and time although thats been debated in the past as well.
I was planning to build the one shown in the attached link, cost was about $200 off from a complete unit shipped (that included having the pieces pre cut by my metal supplier). Since I don't plan on using it much I opted for a good body cart, thats more than sufficient for my needs at a fraction of the cost.
Kron