Got one under construction now, but it keeps getting set aside as other projects keep getting priority. I used 12' 2X6s cut in half then ripped down the middle. Much straighter than using 2X4s, though the top winds up a little over 2" thick instead of the 3+ inches.
Plan is to glue them up in groups of 4, then run the groups through the jointer & planer, then glue up one set of the 4 wides, and run them through the planer, and then joint one edge on the 8 wide piece, then glue them together, total of close to 18" wide top. Then on each side, use one of the remaining pieces to cut a piece long enough to fit between 2X6 legs and a short piece to fill from leg to end of bench.
Last piece is then glued to each side over the 2X6 leg. 2X6 legs will be ripped to remove the rounded edges. 2X6 legs get 2X4 on the back to make an L, 2X4 stringers top front to rear,and near bottom front to rear and side to side. The last 2 2" pieces surrounding the legs will be glued with a few screws until the glue dries, then removed and hardwood dowels substituted. ANyway that's the plan.
Alexis de Tocqueville was a very smart man.