I looked for awhile trying to locate a viable source for a forend strap for my M500, then decided to take on the task of attaching my own. A trip to the hardware store for four screws, four washers, and a nylon strap off of an old backpack I found in my basement turned out excellent results. I have a link to a picture I took on my phone below:
I had one that came with mine. I took it off and replaced it with an adaptation of one of those fancy para-cord wristband things. I figured it was a little more durable, and I could unravel it for emergency purposes.