This is the last section I put together.

ONE

First, I cut a 24" x 24" board down to 17" x 17", to be the drive section where the fan will be mounted.

TWO

Next, centre the fan and trace your circular cut. The fan I ordered came with a template, so that was handy. Just make certain the fan will be mounted in the centre of the board.

THREE

Cut the circle as precisely as you can. I took my time using a jigsaw to ensure as circular a cut as possible.

FOUR

Fit the fan in the hole and make sure it sits well. If you need to open the hole a little more try sanding. If you need to open the hole a lot you may need to use the jigsaw or another tool. Once you are comfortable with the fit secure the fan with screws - the fan I used came with mounting L-brackets and hardware. NOTE: we want the fan to blow the air out of the diffuser section, not in to the diffuser section. Keep this in mind as you mount the fan.

FIVE

Next, cut your 4'-0" x 8'-0" board in to four pieces. They will need to be of equal size and trapezoidal in shape. They should be 12" at the test section end, 17" at the drive section end, and 2'-0" in length. SAVE YOUR SCRAP WOOD!

SIX

Same process as the Contraction Cone, connect the boards together. This time is easier, as you can use the fan/drive section at the base to mount the boards. NOTE: you will need the power supply for the fan to be accessible. For this, I drilled a hole at the base of the drive section big enough for the cable to fit through, then ran the power cable out before mounting to the diffuser boards (visible in video at beginning of instructable).

SEVEN

At the drive section there will be a gap due to the way the boards will mount. This is unavoidable, but can easily be patched with some sealant or lots of silicone. As with contraction cone, run silicone along seam to ensure air tightness.

EIGHT

As with contraction cone, build a base so the diffuser runs along same axis as contraction and test section. I used scrap wood to achieve this. Some measurements should be taken to ensure you are as close to aligned as possible. The diffuser base will be a bit more difficult as both ends will need to be raised, where the inlet of the contraction cone can rest on the floor.