maybe the size of crayola 24 count crayon boxes

Those sound neat. Pine is inexpensive, and isn't so hard that it would be likely to splinter while nailing or stapling it together. One warning, though. Pressure-treated woods (the kind used in construction, and sold in bulk at most home-improvement stores) have chemicals that can leach into the soil and be bad for plants. I don't know if they could also leach into seeds that are sitting on the surface, but I wouldn't risk it, personally. I think it's a great idea, since you'll be able to use them from year to year. I was just thinking about a cassette-tape storage rack I have, and thought the openings in that would be the perfect size... so if you still have any cassette tapes laying around, maybe measure it in the plastic case and see where that takes you. Also, some home improvement stores sell pine board that has warped or is imperfect for really cheap. I've used that (with plastic sheeting and a few hinges) to build a 1-season cold-frame before, and they would probably be a good width for your boxes if you cut the length into shorter pieces. Although my cold frame made from that material only lasted a year, your boxes wouldn't be exposed to the weather. The pine boards that I got were about 2" wide by 1/4" thick, and 4 feet long. 1/2" staples and a staple gun worked wonders.