Cutting a hole would work. Their is plenty of depth.The grille on the door panel has an unusual shape. Make a trim piece to screw or bolt to the panel to cover the original space of the grille, cover it with carpet or vinyl and attach your speaker to that. 1/4" or 1/2' mdf, bb, plywood would work.
Kick panels would be cool too. You will lose the use of the e-brake though. The lever itself is easily removed though if you don't need it or don't mind losing it. I will be competing in car audio competitions this summer and I could lose points for that. That's the only reason I didn't do it myself. There are plenty of how-to articles on how to make custom kicks on diyma.com, roe, stevemeadedesigns, etc. There is wiring for each door and other misc. wires behind each kick. Just FYI Put up some pics when you get them done we would like to see em'