I would recommend 4 or 8 ply for this as well. 8 ply would probably feel sturdier and might help hold the paper flatter. Having a frame shop cut it would be a good idea. They should not charge you more than 4 bucks for it, or might just charge for the board and cut it for free (I used to work in a frame shop and did this all the time). Just beware that this time of year is very busy for frame shops, and they might not be in the best of moods during weekends.