I made some for my shop. Cut half of the 2x4's 12" long and the other half 16" long. Only other items required are top quality wood glue and big screws (long drywall screws). Join each board securely to it's neighbor with glue and screws. Let the glue dry real good (read the directions on the glue container). I went 12" high and wouldn't recommend going taller! Just be carefull and don't you (or your buddies) rock the car when anyone is under the car. You can mess even this up if you try hard enough!
If you are not handy with structural woodworking, go buy steel jackstands instead!
Nothing new with the air dams rite now. I'm in th process of smoothing my bumpers, then I will have everything painted at the same time. I also made some aluminum bumper brackets!! I'm ready to get it back together!!