I am looking for some advice on building a swing to go with the deck (which was done using pressure treated).

My design plan, which is pretty basic, is to take about 4 2x6's and cut them out to form the seat and backrest, and run planks across. The problem is finding wood to use for the planks.

I was thinking of using pressure treated boards that are about 4" wide by 5 feet long. I looked at 1x4 boards but they seem too thin, and 2x4 boards but they seem too thick. 5/4 deck boards seem like a good thickness but I can't find any that are 4" wide, only 6" wide.

One lumber place had 5/4 and 4/4 in Ipe and Brazilian Redwood that is 3" and 4" wide, but it is a little expensive plus I don't know how it would look mixed with the existing pressure treated deck. Also, doing some reading it seems like cupping is an issue with these types of wood.

Would this be good wood to use or should I go for something else? Alternatively, if anybody has other design ideas for the swing, it would be appreciated as well.