Whats the best way to shave the ribs off the inside of the tailgate?

Whats the best way to shave the ribs off the inside of the tailgate? Im wanting to shave the inside of my chevy fullsize tailgate and didnt know the best way to go about this. i was thing about cutting the rib part out but didnt know if the tailgate would sping on me and loss shape. any advice would be help full thanks.
 
 
heres about of what ime talking about doing.
 
 
 
 

just cut out the inside and use a piece of sheet metal.

Thats what i had planed but have you done it? i dont want it to lose its shape.