A few thoughts / suggestions:
Use a yellow glue such as “Titebond” or a poly glue (“Gorilla Glue”) for laminating your stringers. It’s got a longer “open time” (working time before it sets up). Cool down your shop or do it on a cool day to slow down the open time.
To get a 1/2" stringer use three 3/16-" pieces. Redwood, cedar, basswood, especially balsa will bend easily in these dimensions.
Don’t mess with steaming or heating 10+’ long boards.
Don’t use construction grade lumber. Use only kiln dried Clear all heart redwood or similar cedar which you can get from hardwood lumber dealers (places that specialize in furniture grade lumber).
If you don’t have a bandsaw find someone to resaw (rip) your bands preferably all from the same board (boards) to ensure uniform color, grain appearance, grain direction, workability.
After slicing off all the thin pieces, sandwich them back together agin using a couple of finish nails driven all the way through the bundle (as if it were a full 2"x6" x 12’ again and bandsaw out the curve of the rocker on all pieces at once. Cut them to general oversize. This way you won’t have to hassle with grinding down all that lumber, as each stringer will be closer to finish shape of the rocker in your board. Place the finish nails where the rocker will be cut out or where the remaining holes from the nails will be hidden safely inside the surfboard.
An extension of the previous note: When gluing up a few thin pieces together to make a thicker stringer, spread glue on all pieces than take together with tiny finish nails to keep them from slipping and running wild once clamped together.
Jim’s idea of using bicycle inner tubes to wrap clamp everything up is a good one. If however, you don’t have time to run all over town to find enough bike shops to supply you with enough tubes there’s another way. Go to the Depot or the Low place to the garden department and buy a lenght of rubber pond liner sold by the foot off wide rolls. It’s the same rubber as bike tubes and you can scisor cut it into many strips (2" wide +/-) to use for your wrap clamps.
remember 9th grade geometry, or if you’re as old as I am and can’t remembr 9th grade you can draw out an exaple of the fact that you can’t expect to cut a long figure-8 with a thin saw blade and expect to fit the two sawn pieces neatly back together with a 3/4" stringer between them. Since the saw blade is not the same 3/4" thick as the stringer, you won’t get a fit. You can get by with a thin stringer because the soft foam will cheat its way back when clamped. However, expect an increased width in the stringer to result in a forced sloppy fit and gappy glue line unless you make two cuts (one on each side of the stringer).
Just a few thoughts than jumped out that you may want to consider. Be methodical, think through each step, and Enjoy the ride!