I think I understand what you are trying to accomplish, however, if you can find some bricks to line up on the charcoal grate, that would probably be a better long term solution. Also, keep in mind that for longer cooks, you can control the heat by using a small portion of lit coals next to a much larger portion of unlit; the unlit portion will gradually catch fire, and if you work your vents properly, you should be able to easily get 6 hours+ on one load of charcoal if you are cooking at around 250 degrees.
Here's a good thread so you can see some indirect kettle set ups:
http://www.bbq-brethren.com/forum/sh...d.php?t=129246