1. You need a circle jig and a router
2. How much air space is that for the woofer? It looks very small for 3 10's and not even close to enough for 3 12's. Maybe a pair of 10's would be better suited then you dont have to buy another amp. It is very important to the performance of the sub to have the proper amount of air space for it and not just put it in any space that the speaker will fit in. If your goals are just to be louder i would look at a better pair of 10's with more power/ better amp.

As for down firing, I wouldnt worry about it. People do it all the time in trucks where the box is under the back seats. As long as the air has a way to get into the cabin it should be fine.