Funny that you suggested the 421d. It was the first amp i thought of when i saw this thread. Had one a couple years ago that i got for like 60 shipped and for what it was that little amp had a lot of balls.
i got a pioneer gm-d7400m, it is a 400wrms @2ohm momo amplifier. I got it for about 120 shipped and it provides me with everything i need, not to mention all in a small package. It is roughly 9x8x2 and is very symetrical. I have had no heat issues with this amp and it is a very solid amp as well. it does not feel like vibration could effect it much and is very rigid and sturdy. I found this to be a great buy.