S25,

Lots of good suggestions here. I have always found that any material into which it is easy to push pins is too soft to hold them well. Just my personal experience. I use a very light tack hammer (found the one I use now at Harbor Freight) with a light touch to drive T-pins into the board. A balsa building board may be the exception...never used one! It will be real hard to find a wide, pine, plank that does not come pre-warpped. Good luck with your search.

Hope thing go well for your son. I am sure you are very proud of him, and should be. Semper Fi and Semper P.!