One possible low cost option for a open baffle sub is to use a corner of the room and mount a tall board sealed to the corner of the corner of the room. The width of the baffle board must be wide enough to fit the woofer. This would provide an open baffle of very large proportions plus additional reinforcement of the corner loading.If the baffle were sealed to the floor and ceiling and at least 4 x Vas of the driver in internal volume provided you'd have an infinite baffle design with the same corner loading.
Either of these designs could be duplicated on both front wall corners or multiple drivers could be mounted on the same baffle.