I've seen a couple of baggage shelf mods.
As you contemplate this, remember, that aft bulkhead and its cover are major structural members.
One guy put a shelf in across the entire width of the aircraft level with the top of the lower bulkhead cover that went aft to the next bulkhead. He then put a hinge on the upper section and made that so it would close.
Other modifications I have seen go back further or along just one side.
As LA said, what you can put back there is very limited due to CG restrictions. However, the people who have done this tend to carry things like sleeping bags, fly rods, and other long & light stuff. Golf clubs are an issue because they are heavy! If you want it for that, make sure the heads are up against your seat back and just let the end of your travel bag stick aft AND check your CG!
Whatever you do, remember that you have to army crawl into the tail cone for your condition inspection so don't block it off completely.
