Here's how I got the clubmans on. Admittedly, it's a bit on the grunge side of things, but I tend to like that (you should see my car).
The platform on the frame was made out of two layers of 1/8", 3-ply aircraft plywood glued together and then dried while clamped to the frame. Since the frame has a curve, I arranged it so that 4 of the 6 plies are perpendicular to the longitudinal axis of the bike (easier to bend that way). Leather was then attached to the plywood with nylon bolts and a strip of aluminium (a bit of personal nostalgia there, since the leather came off the pants that I bought for my Norton 850 back in the late 70's).
Everything seems to work well. I took it out on the highway yesterday with an Oxford tank bag bungied to the the yellow platform. I felt relatively comfortable for the whole 90 km's, except for a mild discomfort on my throttle hand (I'll have to get one of those clamp thingies that my wrist can rest on, and which emulates a cruise control to a certain extent).
I'm sure someone could do a better job than me with either fibreglas or carbon fibre, and some yellow dyed leather. With a downward lip on the fibreglas, it would also be much more rain-proof. I looked at the Honda CBR1000 fake tank, and although it looks a lot better, I kind of like the flat surface, both for the tank bag, and just because it has proven to be a very stable little shelf to put my helmet on when I'm getting stuff in or out of the givi side bags.
The clubmans are significantly wider than clip-ons would be (even after I chopped 1/5" off each end), but I like the added leverage for avoiding potholes, etc.