Yeah I think milling the edge of the block is the best solution, and it's what I did with my VPU block.
I hope to start making a limited run of VPU blocks soon, especially for the R 9700 / R 9500 that will have both a milled edge to avoid the shim, and a better than usual retaining method, as well as being all copper, soldered construction and shiny.
will probably be very similar to the one I made for the
R300 water-cooling project.