Hiya
There pretty easy, done it about half a dozen times :)
I use 100g Chopstrand matting, 3-4 layers would be fine, also a trick i use to 'brace' the glass'd section is to use some 10mm rope on the 3 layer then apply lots of resin on the 4 piece of mat.
Here's a guide on Talkaudio - http://www.talkaudio.co.uk/vbb/showthread.php?t=76606
Auto Acoustics make them too, but acoustically they don't give you impressive results. IB is probably the best method, also the hardest in the IS200
I choose 4th order vented, tuned to 28Hz
Good luck
:)