I have one on a Nauticam EM5 Mk1 housing and your housing looks the same for strap attachment. You use the smallest 1.5mm allen key to move the metal bar at the bottom of the housing to attach the strap. There's a small hole towards the front of the housing, where the allen key fits into the metal bar to unscrew it. It's tiny, not so easy to notice and is not detailed in the housing instruction manual. The allen keys were included with the housing.
Definitely get the longer strap as I found the standard too tight.
The strap is attached to the housing at the top with two included screw-in bolts that use a larger included allen key. I found the metals can fuse together with the salt water. To avoid this, I'd work the bolts occasionally and lubricate them. Oring grease should help here but there's probably some other lubricant available for marine applications. A Nauticam retailer could probably suggest a lubricant.