I read this post earlier but refrained from commenting as I personally haven't used this particular filter before (one of only a few that I haven't tried!).
Basically you run it with a backplate on and use some extremely short ram tubes such as these:
http://www.dellorto.co.uk/merchandise/p ... ctionID=41
The issue is that the foam literally sits on top of the ram tube with no clearance (flush against the mouth of the ram tube) and this is essentially like stuffing a cloth over the intake of your Weber. For ram tubes of any sort to work they need some clearance around the front and the mouth of the ram tube.
I agree though, without a weber box, there isn't much choice at all. Hence why I made the (excruciating) decision to weld in a box and never looked back...
EDIT: you can try one thing, install the short ram tubes as above and pop one of these over it:
http://www.dellorto.co.uk/merchandise/p ... ctionID=64
Basically it's just a hollow foam "cage" over the top of the ram tubes. Yes it will be a squeeze in some bits, especially the bottom, but at least it will be open and unobstructed sides and above. This MAY be slightly less restrictive than all other options? FYI this is what I run