Mounting a speaker directly onto the metal is doable. It's avoided simply because all that energy is directly transferred to the metal sheet and that creates vibrations and resonance. Even the factory speaker is mounted on a plastic ring which is fixed. So using a bracket is always a good idea. If you dont want to, using a foam gasket is a MUST behind the speaker
Recoil makes a really nice 2 way coaxial (13k), known as the RLX65. Very clear and pretty decent midbass. Low power requirement and high sensitivity means it works well with factory headunits. The component version of the same speaker (RLM65) is for 16k and uses a 1" silk dome tweeter which you can install higher up to really improve the sound stage. Again, works great with factory headunits
The other cheaper option is to get some factory panasonic speakers made for most JDM toyota's. You will need to make new holes to mount these but they sound great compared to the automate speakers installed in pkdm toyota's which then sound better than the crap suzuki installs in their locally assembled cars. Usually the panasonic speakers cost around 4-5k for a pair, the toyota automate ones cost anywhere between 3-4k for a pair
