This item is currently on sale at Surplus Shed (I have no relation with them other than one-time customer): http://www.surplusshed.com/pages/item/l14550.html
I guess one would have to remove the gold coating except on the RF patch, and replace the whole splitter plate at 45°, then recalibrate the RF. Easier said than done, but still easier than cooking a half-reflective coating in your kitchen. Or, maybe, leave the whole plate as-is and have a "full-frame" RF patch.