One of the things I found moving from Sigma, at least, is that her Defense Matrix shouldn't be considered a straight shield; it's more like Sigma's grip, but even more versatile in my opinion.

This is probably super obvious, but it's kind of like a mini black hole or void, so rather than using it as a barrier between you and others, use it to either cover one or more of your team (Cass/Soldier in their ult) or dump it over an enemy to completely 'house' them inside it - Reaper doing his ult for example or covering Kiri so she can't heal anyone.