In this case, the best place to declare the type would be in a namespace that is by default in scope in both Strategies and Indicators. Typically, NinjaTrader support and Development both will tell you to declare the type in the global namespace. (There is a reason for why they do, but it is still not a good idea). As all strategies bring the NinjaTrader.NinjaScript.Indicators namespace into scope, that might be one place where you could declare your new type. Of course, all Indicators reside in that namespace and so will always be in scope.
Once declared this way, you would then simply use your struct as the type that you are declaring in any class. If you put that in a partial class, then a copy of it will be available to anything that inherits from that partial class.
ref: https://msdn.microsoft.com/en-us/library/0taef578.aspx
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