Reply by Bill White

Did ya glue the joints? If not, use some wax fillers such as SoftWax by FastCap. I use this stuff for trim molding, etc.
You can fill after finishing if ya want. Rub it in, buff it off. You may have to reapply from time to time. It is just the nature of unstabile miters.
I often use biscuits to help this issue on new builds.
You didn’t state what type top you built. Expansion and contraction will often be an issue with miter joints.
Bill

-- bill@magraphics.us