1. An impact driver is not necessary, if you are using a lot of screws which would be considered optional it is very handy. When using MDF do not forget you need to run pilot holes that are roughly 80% of the diameter of the threads of the screw you are using. I found an aluminum 48 inch ruler that I was planning on using as a guide. You cut across the sheets first instead of up and down so the ruler will reach.
2/3. From PE (no harm rounding to 20 inches) I spent the money on a carpenter compass, because I botched the first hole without it. I think it was around $10 on Amazon. Just make sure it has a radius bigger than 10" if you pick one up. Triple check measurements for the cutout. As an example if you are 1/16th" off on measuring the radius, it turns into an 1/8th" off on the diameter.
- Overall Outside Diameter21.54"
- Baffle Cutout Diameter19.97"
- Bolt Circle Diameter20.67"
- # Mounting Holes8
4. A brad nail gun is optional, but it can help speed up the process of assembly. Titebond II can be used, but it does not fill voids as well as PL3X. John LTD02 has tested the strength of the PL3X, it is strong than the 2x4"s it attached. You will want to pick up a few F clamps 36" or better. I have a few C clamps and also Irwin handi clamps for holding things in place. Just remember it needs to be air tight. If you are going slow a lot of clamps is a good path to take.
5. The cutout would be done with a jigsaw on those front rails. 1 inch deep is perfect. Check the LFE, I have the length of the cuts and the distance offset from the top of the rail in there if I recall.
6. Dampening material is not called for. The design is really meant to crossover at a maximum of 100Hz.
7. Since the size of sheets have shrunk you will need three sheets now of the material listed above.
8. You can use those 2x4"s, just make sure they do not have a whole lot of bow in them. If they are all really bowed there are 2x4" studs that are more expensive that are meant for cabinet walls where you really want straight lumber. They are jointed together and not one continuous piece of wood which could be useful for the long vertical braces in the back.
9. Starting a build thread and asking questions as you go along is a good strategy