I bet your just like myself, whenever your out at a store and look at something you will think to yourself " i can use that in my project!" even if said object wasn't meant for any such use! Just like you have done with the spikes...simply brilliant!! I have seen fence "decorations" that would go close to fitting that larger tube, the type that slot in on top of fences to "deter" people from jumping them, you can get them in all type of styles but i have seen "pointy" types that would fit the "theme" might be worth a looksee into...Alternatively one you no-doubt would have thought of already, hit up a machine shop and get a custom alloy 'spike' made...
Best of luck anywayz....superb build.
KiM