The Night Ranger Quad really should be a Cobra Ferret.  With a few modifications I went ahead and made one myself.  Here are some work in progress shots followed by the finished product.

I started with the Sigma 6 Night Ranger Quad.  I took a Duke v.1 grapple cannon that I had removed the guard from for another custom (Road Pig).  I removed the grapple hook and string.  I used a 1/6 scale mortar launcher I got as a freebie from One Sixth Depot and cut it down to just the barrel.  Luckily the inside of the barrel fit perfectly on the end of the grapple cannon.  I had to trim down a few of the weapon clips on the Quad's rack to get the back of the new cannon to sit right.  Using the string cap from the grapple cannon as a mount I superglued the cannon to the rack.  Then I used the elevator on the barrel and glued it to the body of the Quad.  This makes opening the front missile launchers impossible now but that's okay, its not something I'd do with this vehicle anyway.

Taking a Joe AGP that I had already cannibalized for parts, I cut off the wing tips to add to the Quad as missile launchers.  I trimmed them down to fit and so the front wheels wouldn't rub and glued them to the bottom of the footrests.