Fence boards are usually a nice 1x6(not precisely). I’ve used some for picture frames and when routing the edges for joinery and actually holding the picture they’re easy to work with and also sanded down to nice smooth finishes without much effort or sandpaper. Plus they’re light

Oh and a 1x6x6 treated fence board for $2.50 beats the crap out of the price of its competitive lumber options $8-??