The built up targets that I've made were done with smaller cookies with a 90 degree corner put on them. Join 4 and you have a sort of cloverleaf shaped target. It seems like a 20" or so log is required to get a good target doing it that way. We toe screwed the 4 quadrants together and then put a piece of ply on the back (1x's would work) and screded through it into the back of each section several times. you could also do that with timber cutoffs.

I wouldn't paint or seal it, though, if it will be in the weather. My experience is that wood lasts much longer in the weather if it can dry out quickly after it gets wet. The paint/sealer only traps moisture, facilitating rot.