Has anyone drilled holes in the floor of your max to fit universal brackets? If so, how do you keep the bolts in? with nuts underneath the car?

Don't...you can buy custom brackets from Wedge metal works.

doesn't wedge make them for corbeau?

I know they make custom seat brackets for maximas. You can pretty much fit anything under the sun. ie..Momo, Sparco, Tenzo R, Recaro.

I know mobiltek sells them, but is there any place cheaper to get them?

If you check some of your local shops, one of them might be willing to actually weld on a custom bracket (if it's easy to do, it will be cheaper too).

As far as I know, no one makes holes in their floor to custom fit seats. Rust and water