becuase with 2 6.5" drivers 3/4 is not necessary especially ported.why half inch
just do the math. find out what the internsl measurements are suppose to be. Like in those dimensions, if you used 3/4'' it would be 1.5'' off of each measurement. But, if you used 1/2'' like you want to just subtract 1'' of off total dimensions just, make sure you keep the internal dimensions the same