Jon Jr. is a small jonboat, sort of a personal sized boat that will take two normal sized adults. It would be a perfect "back of the pickup truck" boat. It could be cartopped. (To me the idea of cartopping a powerboat is a bit strained since you would need to carry the motor, with its smells and drips, and the gasoline inside the car). If you are sure from the start that you will be trailering the boat, you might consider the larger Jonsboat design which has more capacity, but is not so large that it will be expensive to build, use and haul.
As for power, I'd suggest 5hp max. Sometimes you can find low power motors at yard sales since they don't fit the image of modern boating. The usual rule I've seen for power on planing boats is a horse for each 50 pounds and motor salesmen like to quote a horse for each 25 pounds. I've found that in good conditions a simple flat shape will plane with less, so this boat with a normal sized man might plane with 5 hp. Actually, the Coast Guard suggestions for powering boats comes down quite hard on small hard chined boats like this and they might advise even less than 5 hp. A boat like this is unsuited for big waters anyway. So the little motors will cover the little lake quite well.
Construction is of the simplest "instant boat" type from three sheets of 1/4" plywood. All nail and glue construction with a smattering of epoxy and fiberglass to fortify the chine corners and help seal the water out of the bottom joints.
The prototype Jon Jr. (below) was completed by Joe Leinweber and Dan Ellis and tested to a certain degree.
JON JR, POWER SKIFF, 12' X 4', 100 POUNDS EMPTY