I made my own out of pressure treated 2x6.
Cut to length. Run one edge through a table saw so that it is reasonably square to the face. Use a speed square an make 2 or 3 lines across one board. Us the lines as guide and drill a 1/2" hole about half way across the board. Apply construction adhesive and 4" long screws to connect the two edge faces. Make sure the growth rings are going in opposite directions. By making your 2x12 this way, it is a lot less likely to crack !
You can do the same with PT 2x4 but you will need 3 pieces. I have found that they might seem heavy at first, but leave them exposed to the weather and they will be a lot lighter the next year.