This bag can comfortably house TWO HL-X18's if they are used with the (optional) U bracket for mounting onto a stand, rather than with the multi-mount bracket. There is quite a bit of room in and around the bracket when using one light with MMB.
The bag itself is very durable and well padded inside. The zips can be somewhat sticky when new, but a quick (light!) spray of WD40 along the length solves this quickly.
The shoulder strap along with the normal straps is a godsend, as with two lights, plus brackets, plus cables in each of our two bags it can be quite heavy.
In short - bigger than you might expect, but very fit for purpose.