Lot of variables, but I'd look for a used Harley trunk rest (probably the cheapest and most numerous you will find) then see if you can figure out how to mount it to your trunk. The trunk (or lid) would probably have to be strengthened to hold the load, and the trunk rest itself may have to be reshaped on the back (mount part) to mate up with the trunk or lid (an upholstery shop could probably help with that).
When you are done, it may be nice for the passenger, but probably will not look very good.
You would probably be better served to get a complete used Hog trunk with matching rest attached (much cheaper than used OE valk parts) and then spend your time figuring out how to mount that assembly to the bike's luggage rack.
https://www.denniskirk.com/397031.sku?utm_source=google&utm_medium=cse&gclid=CODSzL7vo8ICFfPm7Aod7ScAcALook at all the sizes and shapes.
https://www.google.com/search?q=used+harley+trunk+rest&client=firefox-a&hs=8vd&rls=org.mozilla:en-US:official&channel=nts&source=lnms&tbm=isch&sa=X&ei=YNx7VKD-M8GZNrzRg_gF&ved=0CAkQ_AUoAg&biw=1412&bih=652Ebay.
http://www.ebay.com/bhp/harley-tour-pak-street-glide