The cheapest and possibly easiest way is to just get a second hand tank. You should be able to use glass or acrylic but I think the silicone will stick better to glass (I could be wrong about this). The thickness of the dividers doesn't really matter because you aren't trying to hold in water, just direct the flow, and there is typically equal pressure (or close enough) on each side.
Don't forget to put in a bubble trap before the return!
Also, are you planning on having a skimmer in there? Best to put that in the intake section if only 3 so that the water level stays about the same, plus any bubbles produced will be handled by the bubble trap.