I made my base by laminating together a pair of 2" x 8"s; you can eliminate this problem by using a regular 2" x 12". That said, lamination is a great way to make use of smaller pieces of scrap.
Rip a straight edge on either side of your boards on the table saw. Go ahead and glue up the whole 8' lengths of wood; you'll cut them to length later. Generously coat with glue (I use a mixture of wood glue and polyurethane glue like Gorilla for maximum strength) and clamp together, cleaning up squeeze-out with a damp rag and putty knife. Once the glue has cured, run through a thickness planer or use a hand planer to clean up the seam.
Run a rabbet
on one of the long edges of your newly laminated piece (or 2" x 12", as the cse may be) that is 3/4" x 3/4". You can use a table saw for this, or set your circular saw shallow and use an edge guide to make a straight cut.
While you have your saw settings, go ahead and run the same rabbet on your 2" x 4", for the back supports.
Cut the rabbeted 2" x 12" down into two 36" pieces on a chop saw.
To make the wedge shape, clamp the 36" blank to the edge of a table. Strike a line on the non-rabbeted edge that tapers from a full 11"-12" on the one end to 4-1/2" on the other. Cut that line witha circular saw. Your rabbets eventually have to face one another, so make sure your tapered cuts are mirror images of one another across the two pieces.
To increase the long-term durability of the bases, which are made of soft wood, I put hardwood runners made from old oak flooring on the bottoms. Cut strips of hardwood (oak, walnut, maple, etc.) to 1-1/2" wide, then glue and screw them to the bottom of the wedges, planing the seams smooth.