Step one) First build your box. This particular unit is built to accept standard 16 x 25 x 1 inch furnace filters. I choose to use only scraps for this whole project, which dictated the depth of the box. You may want to be more deliberate in the cubic space of the inside of the box and the amount of pressure the blower is creating. I used 3/8 in ply and attached some 1 in strips to the top and bottom panels to allow to sides to be glued and tacked on . The back panel then fits into the the back of the box ad can be screwed into the ends of the one inch strips (do this last, so you can access the inside of the box and to complete the construction.) You can glue and tack these parts together or screw it together. See the pic of the front of the completed box .
Step two) Now we need to build some cleats to attach the filter. I used some fluted aluminum sheet. This makes it look real classy Any sheet metal close to 16 gauge will work. I cut some strips at 3 in wide and put a 90 degree break in it to create a hook. Then drilled some holes approx. 1/2 in from to edge on to 2 in side of the cleat. Now using some shot screws, fasten the angle piece on to the box leaving enough room for a filter to slide in and out of the frame. you could use the filter as a spacer or measure 1 in and a 1/8 away from the front of the box. once you have at least two cleats on slide in your filter to test the fit. I also made my filter but that is for a separate instructable, coming soon.
Step 3) Now time to do the back side, draw a 4 in circle. Cut it out with a jig saw. Now use a male to male 4 in duct connector, cut some slices on one side and peel them back. See photo of my 5 minute flange. Screw the peeled back areas to to inside face of the ply wood. Then attach some 4 in flex tubing with a hose clamp to the other side. Do the same to the blower side and you are donzo. You can make up your own mind about what blower to use I just used a Harbor Freight, cheap boy so do the math.