Keith,
I also had this problem.
I did two things - one I found a water bottle holder with a flat mounting bracket. The inexpensive ones all seem to have a little bit of a curve to them.
This one, the Specialized Rib Cage Pro Road
http://www.artscyclery.com/viewlarge.html?pcode=SWCRCP&large=BK
Is flat.
Next I had to find a better nut/bolt solution because on the regular inexpensive water bottle holder the bolt was always interfering with my bottle.
So I found these things, not sure what they are called, but they are kinda like self clinchers? They are made out of aluminum and the male part is just a very flat, shallow headed standard screw while the other side looks like a pop rivet but with the female thread hollow all the way through.
Here is a "drawing" of what I am talking about.
The holes in the seat pan aren't big enough to allow the female part through but the cage bracket is and it doesn't matter because the 3/8 inch size doesn't reach to the seat pan from the far side of the cage bracket anyway.
I also glued a small piece of innertube between the cage bracket and the seat pan - just don't like to scratch my bike.
My bike is at my LBS today - I will add pics to this post when I get it back.
Haven't had much of a chance to test it but it looks like it will be rock solid.
Oh yeah, and then the bottles I put into these cages are what I use for my tools for commuting.
I had never heard of Silca before and here is a site where they have a "tool" bottle.
http://www.silcapompe.it/borracce_en.htm
But mine look like this - I think they will work dandy.