It seems to me you are looking for a boundary microphone, also known as a PZM.

Radio Shack/Tandy/Realistic used to sell one, made by Crown (owners of the PZM trademark), which was quite cheap.

You can make your own, however, by fixing a tieclip microphone to a polished board (plastic would do, or even metal), which is what we used to do before the manufacturers got wind of the idea!

AKG do a C562 'ceiling mic' which is ideal for the purpose - it was designed to screw into a hole in a board... it is, however, 'phantom powered' - needing at least 9v DC from the mic input, or an external power supply.

You can make it cardioid by lying the mic on its side, or omni by poking the mic up through the middle. Experiment for the effect you require.

As to the SM58 - it is an excellent close vocal mic (industry standard), and has a 'peak' in the midrange to help eliminate feedback. The sound is, therefore, 'coloured' when recording - I'd avoid it, sorry! Although it's good on drum kits...

As to EQ, etc. You are far better off recording 'flat', and doing any EQ or other processing on the mix down. Once it's recorded you can take it out, but it's hard to put back!!

Boundary layer microphones pick up direct and reflected sound, and noise is rejected by phase cancellation.

This type of microphone is good, for example, on a church altar, as it can be used by a number of people simultaneously, or by the same person moving around the altar.