If you re going to cut corners on your effort I would suggest NOT to do it with the very foundation on which your RRing is being built. That approach may very well reach out and bite you.
Cross members on 16" centers will allow most hand tools access for drilling holes or driving fasteners.
I would avoid pine plywood decking. A sheet of plywood will yield thirteen 3 1/2" X 8' frame material. Take the price of any quality plywood and divide by 13 to get your unit cost.
Dimensional lumber will warp. It is only a matter of time and severity.
This works for me (one of them survived intact from a tailgate drop @55mph) :
