i just built a couple of sandwich boards yesterday using compressed pine...im not sure what the technical term is, but its like 5 layers of pine compressed together. Anyways, i'll let you know how it holds up. I'm curious to see how strong it is.
Sure pine is okay. I use boxes from pine and whatever wood I can find for free. I build it and then jump on it with my feet to see if there is any flex. Brace it more if you have some give in the boards. On your uni the weight is in a single area as opposed to standing on it with your feet.
I have some boxes with a 30" span for the pine boards that I added a piece of 2 X 4 underneath for extra support.
Go for it! Never throw out wood, it can always be used.