A small insert doesn't collect dust as well as a large insert because the small opening restricts the cfm. But a large insert doesn't support the work as well. Is this correct?
I was thinking that the insert behind the bit doesn't support anything, so opening up the insert there would increase the cfm and improve dust collection; make the opening an oval rather than a circle. Does this make sense?
Assuming you are with me so far, would it be reasonable to make this oval by screwing the aluminum insert to a block of wood, and going at it with a hole saw? The material left could be cleaned up with file. Or possibly a router? If I don't mind maybe dulling the bit, is a carbide router bit safe on aluminum? I know I once erred with my omnijig, and the router cut into the aluminum pretty effortlessly. I just don't know how safe it might be, again holding the insert screwed to a piece of wood.
If none of this makes sense, I broke a rib last week and the muscle relaxant doesn't agree with me...