Bandwidth Report for multiple WAN instances

Apologies for taking so long to get you answer.  

I would use the Device Metric Trends report for a timeframe you are interested in.  This report is much more customizable than almost any other, and you'll be able to pick out each interface you want to use.  It supports glob matching too, so if your interfaces are named accordingly you might be able to isolate them with a pattern.  If not, you may need to specify each interface... which could require some manual effort to create the report.   

To your point of wanting to minimize manual effort (hooray for automation!), it might be easiest to isolate the WAN interface per device using a special datasource that filters just for that specific interface (by description, or something similar).  Then you could simply make a Device Metric Trends report that doesn't need instances specified.  This solution also has the advantage of letting you do global alert thresholds for your WAN interfaces.  Probably you have already tuned these but just in case, this would likely be easier if multiple devices are involved.

I assume you want to see actual bandwidth and not circuit utilization?  If it is circuit utilization, that is different than interface utilization.  We have a way of limiting reported speed for circuits, so please reply if this is what you're looking for.

Hope this helps,

Chris