I purchased a trunk on eBay for around 60.00 and retrofitted it by matching the attachment points from the luggage rack and using spacers that I cut from steel hollow rod that Home Depot carries.
My wife felt the same as yours with the backrest and the trunk comes with a wraparound seat that when my wife got on it yesterday for the first time gave it two thumbs up.
The trunk comes in gloss black matching my bike perfectly although the paint job is as would be expected...depot carried M8x70 and M8x50 bolts,spacers measured to match were cut at 1 5/8" and around the 2" mark. This gets the trunk relatively level when attached. I placed two sheets of steel under and inside the trunk sandwiching it in order to stiffen the installation. The key point to all of this is that this assembly is removable. That was the attraction to the Harley I owned...detachable backrest. It'll take me about ten minutes to attach or detach, well worth it to have a happier wife on board. Attachment 16250Attachment 16251