The original poster is probably done with this by now but, for the benefit of any who follow:-

Custom Document Properties are visible to and can be modified by any user via the 'File' tab then select 'Info'. On the 'Info' page look for the 'Properties' section which has a small drop-down control. This control provides an option called 'Advanced Properties'. Select it and a dialog opens that contains several tabs. Select the 'Custom' tab. You can now inspect, create and edit any Custom Document Property. Most basic and non-inquisitive Excel users are unlikely to stumble on this.

Note that - The 'Properties' drop down is locked and inoperable if the Workbook structure has been password protected.

If you want to hide things better than that then 'Custom Properties' (CPs) are the way to go as they are, as far as I have been able to discover, only accessible programmatically. It is important to understand that Custom Properties are properties (children) of a specific worksheet - as selected/specified when the CPs are created. If that 'parent' worksheet is deleted then the associated CPs are lost. I recommend that you have a dedicated worksheet specifically allocated for this purpose and hide it to avoid user instigated harm. I usually use a single ellipsis character "…" as the worksheet name but that is up to you.