It is difficult to line up a back exactly with a top - so you might want to consider some sort of design factor to give yourself some wiggle room.
What size are your tops?
What about doing Courthouse Steps - that way you could have the same colors on top and bottom?
If I was going to draft a pattern - I would start with the "finished size" - and start adding strips from the outside edges to the center - and then whatever I had "left over in the middle" - would be the size of the center/middle rectangle (plus seam allowances) - and then add "extra" to the outside pieces to allow for a bit of shifting -