I have never tried something like you suggest, but I have to great success used instead of the carving bit used the 3/16 cutting to hollow out 2x8s to a depth of over .5" and it really didn't take long I thought. I changed your pattern to the 3/16 bit and uploaded it. Optimal was 15.31 minutes. Normal was around 11 minutes. You might want to leave it on normal, as if you have ever seen rough cut lumber, it does have wood fibers exposed. My house was built in 1888, and a number of years ago, when I did some remodeling and updating I wish I would have saved more of the actual old 2x4s and 2x8s that I tore out. I do have several coffee cans of assorted square nails however. Don't know what I will ever use them for, just cool to have! Good luck.
Never mind, I see what you were getting at now, I had a Brain F*** (comes from working nights, and not getting much sleep yesterday) I did a copy and paste of your original design after making the board 144". Try the 3/16 bit on your 24" long piece and see if it doesn't speed up the carve time. I know it works for me.
Hundreds attend Global Warming Protest!!!