I went through this when doing the install in my Ferrari. In an effort to find the thinnest amp(s) possible I came up with a brand called Helix. They are a very high quality german made amp. I've got two of them in two different cars and can strongly reccomend them.

http://www.theautophile.com/products/HelixAmps.html

I have the HXA400 MKII in the Ferrari, and a HXA40 in my SO's Miata.

They are about 1.25" thick if I remember correctly.

No afilliation, yada, yada, yada, just a happy consumer.

Jeff