This module can be used as follows:

 import XMonad.Layout.GridVariants

Then add something like this to your layouts:

 Grid (16/10)

for a 16:10 aspect ratio grid, or

 SplitGrid L 2 3 (2/3) (16/10) (5/100)

for a layout with a 2x3 master grid that uses 2/3 of the screen, and a 16:10 aspect ratio slave grid to its right. The last parameter is again the percentage by which the split between master and slave area changes in response to Expand/Shrink messages.

To be able to change the geometry of the master grid, add something like this to your keybindings:

 ((modMask .|. shiftMask, xK_equal), sendMessage $ IncMasterCols 1),
 ((modMask .|. shiftMask, xK_minus), sendMessage $ IncMasterCols (-1)),
 ((modMask .|. ctrlMask,  xK_equal), sendMessage $ IncMasterRows 1),
 ((modMask .|. ctrlMask,  xK_minus), sendMessage $ IncMasterRows (-1))