After more searching on the net, I've found a few pictures of people doing just what I described.  So I ordered the g spot carbon filter.  It should be here in 2 weeks.

One of the pictures I found had a carbon filter that was pretty close to the shape and size of the Roor filter.  I guess the bulb shape doesn't really affect the vaping.

One person said it worked great.  Another said that you have to be careful not to combust the herbs.  However using the GVG incorrectly can cause the herbs to flame up too.  If any thing I'd think that the extra room would provide a narrower error margin.

Now I need to order a glass on glass bong.  Any one have suggestions for the size?  I was thinking about a 12-14 inch.  Do any of you have experience with smoking DMT/Changa through a bong with percolators?