Specific Aspect Ratios Shortcuts for Lightroom

Hello,

I would like to be able to crop a selected photo to a specific aspect ration with a keyword shortcut. For example:

4x6=Shift+R+1

5x7= Shift+R+2

8x10= Shift+R+3

And so on. This would be faster de hitting r and then selecting the crop I want with a mouse. This way I could also assing the shortcut to my Xencelabs Quick keys or somehting like that.

Thanks,

Jose