Create a custom entity in OFSC

I want to create an entity for a custom extension that will receive and send data to our main scheduling system. The new entity sits under the Activity entity and can have many records for a single activity. Think of it as a Tech going to work to a site on multiple machines. Each machine has its own ID and attributes. Tech has the ability to manipulate these attributes while debriefing an activity. So far - when ever we have had such scenarios - we have created multiple properties under Activity entity - user_comments1, user_comments2 etc. I dont think we