The three white circles are the "flap valves" that allow air into the bottom chamber of the bellows.  I used some scraps of vinyl glued to pieces of thin paneling (circa 1970's) and fastened them down with nothing more elaborate than a staple gun.  You need to remember to incorporate some kind of tether on the flap valves, otherwise you risk the flaps flipping completely over into the bellows on every stroke. Essential they become stuck in the "open" positions and you cannot get much air out of the bellows if the valves won't close.  Yeah, been there...

      The bellows are fastened to the rack by a threaded rod that runs across and under the center board of the bellows. The rod does double duty - it holds the bellows up and fastens them to the racking system.