Will definitely be cheaper than an aluminum frame, although still not that cheap when it is all said and done. Figure on: $30 lumber, $50 for the oarstands, $25 for oarlocks, $10 for hardware, and $10 for finish. As far as plans, just use 2x10 lumber for the sides and the seat cross piece and 2x4 lumber for the cooler bay cross pieces. You can either make two cross bars spaced appropriately for your cooler, or use just one 2x10 that your cooler will hang from underneath (not as useful in my opinion).
Spend some time and get your lengths and cross bar spacing where you want it. I would blow up your boat and spend some time sitting there thinking about it, with a beer in hand. As far as bolting it all together, use carriage bolts with counterbored holes for the nuts and washers on the other side. Buy your oarstands and lay it all out before you cut the lumber so that it will all go together as planned. Bolt her together and then use some sort of waterproof finish. I have used MinWax Spar Urethane with good results. They also make a more durable, harder finish but it is considerably more slippery when wet.
All in all, If you take some time and measure twice (or three or four times) and cut once, you will have a nice frame when you are done. PM me and I can email you some pics of my frame with some detail shots of various parts. Good luck, but you won't need too much. Pretty easy project if you are reasonably handy.