When I bought my house I realized that I needed a bed, or rather a better bed, but I couldn't fit a normal box spring into my car, and I wasn't about to pay for delivery, so I did what any sane person would do, built my own. If you want to build your own here are some simple instructions and a material list to go along with the video I made of the process. These plans are for a queen sized mattress, but can be easily tweaked for different size mattresses.
The support for the mattress is made out of two eighty inch 2x8s that have an eighty inch 1x4 affixed to the lower inside. I also added a 2x4x80 to the center running lengthwise to add additional support to the center. In lieu of a box spring I just used a slew of sixty inch 1x4s laid across the frame, you can see it in the video. :) You could probably opt to using plywood if you wanted, but I had an excess of 1x4s and no spare plywood, not to mention I could fit a sheet of plywood in my car either. Everything is assembled with glue and wood screws. I added two night stands (not pictured) that were made out of 1x4s. They just connect with shelf brackets to the headboard. I finished everything with some stain and two coats of clearcoat.