The proper spike would be the common smooth cone Prussian officer one. The fluted/crimped top you have is for a Bavarian officer helmet. The Bavarian officer has a cruciform spike base while your shell is punched for the round base Prussian. Looks like you have all the parts except for an officer Reichs kokade and a set of chin scales. Regarding the spike you are better to buy a complete one..spike and base, as one top cone does not fit all spike necks. The threading and size of the top screw post varies so one size does not fit all. I have been doing restorations for decades and if you need help or have questions, feel free to contact me via PM.