Need advice for building battery pack for electric skate board ( Razor E-300 )

I am not concerned about how fast the battery’s charge over night is fine

I would just like to make the run time increase by a large margin and try to do it economically

Packing that compartment full of pulled laptop battery’s for 50 or 100 dollars
sounded great to me but sounds like the consensuses is that’s not a good idea?