Need width-adjustment mechanism

Our "shelf tender" grocery store merchandiser presently comes in three fixed widths, 2.10", 2.70" and 3.70". (See at [login to view URL]) We now need to make it width-adjustable. The base is presently one part (in 3 different sizes). We want the width-adjustable shelf tender to adjust between 2.10" and 3.70". The width-adjustable version will likely feature a base with three "strips" -- left, center and right. The center strip is 1.00" wide (and holds the puller strip), and the left and right strips are each 0.50" wide. So, when closed, the shelf tender will be approx. 2.10" wide. It needs to open to 3.70" wide, and we need a mechanism that will enable these parts to open and close evenly, possibly with indexing. We envision 5 total settings: closed setting (2.10"), then 2.50", 2.90", 3.30" and 3.70". We are looking for ideas on how best to achieve this width adjustability with the simplest, least expensive design. I will pay be the hour, but will pay a significant bonus -- at least $250, maybe a lot more -- if the design is excellent.