To increase your thickness without haveing to add more chop mat you can buy a can of the 404 thickener from a West Marine store, it's like $11 for a can and is enough to mix with a full gallon of resin. It's a powder you add to the resin the thicken it up. After cutting out the opening for your sub just mix the powder with the resin to a nice thick consistency then add your MEKP. Pour it into the enclosure and start spreading it around paying attention to the corners and where the sub will mount to. Do this in several layers and you can make your box up to an 1" thick. I did mine to 3/4". This is a lot cheaper and less of a PITA than adding layer after layer of chop may. Not too mention you don't have to worry about it looking pretty since it will all be on the inside of the box. just my .02.

Brian