I've found that Menards has the best planks / boards around my neck of the woods, Lowe's has the worst, but they also have a better selection of MDF... wacky..
You should consider using 1 x 4 instead. They are thinner and therefore easier to bend back into shape. I used a box type construction and have found it to be very sturdy..
I could probably attach a picture if you want a better description, but basically the frame would be made by joining two boards on their long sides forming the corner of a box. This is not only very strong when glued, but the opposing boards actually encourage each other to keep straight.
Yes - the MDF does tend to get chipped easily, but that's why the tmolding is used...