Aftermarket carrier for top case of R1250RT

My wife and I are planning a trip this June through the Midwest for three weeks and am looking for an accessory carrier to put on top of my top case. I have seen the hard Lexan on top of both top and side cases to allow a little extra luggage to be fastened too. I am only looking for the top case as I need to limit luggage but going on three week trip I also need to be prepared. Does anyone have any experience with this type of accessory or have another recommendation? Any suggestions or advice would be greatly appreciated.

thanks

Tom