with those dimensions, with my calc (it's similar to amatt's, but i got it from somewhere else) i also came up with 21 hz for tuning. BUT...if it's a separate chamber box, the length should not be the full length of the box. it should be for the chamber that you are tuning. so... change the length of the box to 24.5, and i come up with a tuning of 35.26 hz.
also...that gives each chamber 4.7 cubes of air after displacement