generally I recommend a silicone case such as those from Capdase,Amzer etc
but for larger phones , I think a hybrid/TPU case is safer even if a bit bulky
Amzer Shock Proof Case for Microsoft Lumia 640 XL - Amzer : Flipkart.com
If this one looks too bulky check out a thick Silicone case only
itll maintain the original look and feel and wont be bulky
Just avoid flip cases,shells etc and usually any other case will do