I've actually been wanting to try this for a little while, but I've taken one of the background layers from treetop town, and edited it slightly so they can stack vertically. I forgot which treetop colour I used to create the level... and I guessed wrong the first time, so that's why I made the white one, too. I'm sure that this will work for a background, but I'm not sure what everybody would prefer to use if they use my custom level.
In the future, I'd also like to attempt to make the furthest background able to be stacked. I'm not quite sure when I'll get a chance to try the furthest background, but I promise I'll try.
Because of my vertical treetops level combining walkway elements, I actually prefer the DKC3 forest background, because the straight trees are... as strange as it may sound... makes the level too vertical. But just in case, I'm going to leave it here. Maybe if anybody else makes a vertical treetops level, or if I make one, somebody can use it.