iPad problems with T-Mobile Internet

I think I've found the problem.   It's the plastic iPad case that is interfering with the wifi signal.  I removed my case and went from 7 to 8 lock ups per day to ZERO for 48 hours.

I cut most of the back of the case out just leaving the bumper around the iPad edges.  So far seems OK.

Something so stupid to cause such a problem.  Plus why make cases that block wifi signals?  Bad design, surely there is material that is neutral to wifi signals.  So far have not found one advertising itself this way.  More then likely each iPad generation has a different antennae location so one case my work and another won't depending on the generation.  Mine is the 9th.