I strongly agree that you should substitute numeric values in place of the (very) long varchar key columns: [SubscriberID],[UserID] and [DeviceID].
But, until then, based on what you've posted, you should very likely cluster the tables as follows. (Yes, forget clustering on the identity column).
For best possible performance, it's very important that both indexes have exactly the same order of columns.
SQL DBA,SQL Server MVP(07, 08, 09) Prosecutor James Blackburn, in closing argument in the Fatal Vision murders trial:
If in the future, you should cry a tear, cry one for them [the murder victims]. If in the future, you should say a prayer, say one for them. And if in the future, you should light a candle, light one for them.