I would start by checking out:
http://www.ampguts.com/ Although, ampguts used to be free and open to view, they may require registration. If you don't want to do that, just google for images using whatever search terms you can think of related to car amps, heatsinks, SMPS, fan-cooled, power amps, etc.
Look at a variety of amps from different manufacturers and see what looks like something you can build based on your skills, parts you can afford to build, etc. You can pretty easily understand the amp's topology by determining the two main amp sections: The power supply and the amplifier. You can actually divide the amp into two sections if you want as well: The power amp and signal processor. You need a chassis to hold everything secure and to protect all of the components. You also need a heatsink to remove heat from the power components. And often times, both the chassis and heatsink are just one piece of extruded aluminum. The power supply is the easiest part to spot as the power transformer gives it away. The power amp can usually be spotted by the emitter/source resistors near the power transistors. And of course the signal processor includes the input jacks, gain/crossover pots, and of course, the op-amps.
Take a look at Xtant and Adcom car amps for good designs of fan-cooled amps.
