When shooting events, now with mirrorless cameras, I normally use a 2 camera strap called a MoneyMaker and had been keeping spare lenses and batteries in a bulky shoulder bag designed for DSLRs. I wanted something smaller and more comfortable which was designed for mirrorless gear. After searching the internet I came across Cosyspeed and their Camslinger bags which seemed to be exactly the solution I was after.
There are two sizes of camslinger bags, the 160 and the 105. The 105 is a little smaller and is shown holding a mirrorless camera with lens attached, the 160 has enough room to also hold a second lens or flashgun. I ordered the 160 and was pleased to find that once expanded it could easily hold 3-4 prime lenses and batteries for my E-M1 with room for a flashgun or 3rd body on top. The separaters are sized smaller for mirrorless lenses making access and positioning much easier than with a DSLR bag.
The bag is based on a belt which sits very comfortably on the hips with the bag lower down at hand level. This is a really great design as it makes the bag very easy to access, but more importantly for me, it sits below the jacket line (depending on jacket length) so when I wear a blazer or suit jacket it does not interfere. I have used the camslinger at a number of events now and it has been a pleasure, it is comfortable for long periods, holds just the right amount of extras and functions great, allowing me to pull it round, change lenses/battery and slide back round out of the way.
I would highly recommend the camslinger bags to anyone using a mirrorless camera system.