The three legged designs are quite stable
Here is what I would do.
Find a design for the top that you want by looking at some of the comercially available designs. An L shape with rounded corners is pretty standard.
Get your friend the woodworker to buy you a sheet of either 1/2" or 3/4" Baltic Birch plywood. They come in 5'x5' sheets so split it down the middle, double it up and cut out the design. Seal with exterior paint or poly. Make 3 harwood wedges at desired angle app. 5"x5" and glue them to the underside of the table where the legs should go. Get 3 pipe flanges and 3 pieces of threaded pipe app. 30'' in length. Set the flanges on the wedges and drill holes all the way through the wedges and top. If you want to countersink the carriage bolts, and I would, drill 1/8" holes first, flip the top over and countersink carriage bolt heads with a 1" forstner bit, then redrill with the appropriate sized drill. Bolt the flanges and screw in the legs. The plywood will cost you between $17 and $25. The flanges are about $3@ and I don't know what the pipe will cost. The Baltic Birch Plywood is the best for this application because there are few if any voids and the multiple layers. 1/2" is usually 9 ply and the 3/4" is 13 ply. Real stable stuff. Because of the the multiple layer it shapes and sands really well. I would round the edges with with a router bit then sand until smooth.