I honestly believe I have the best tunnel extension. I know this sounds like bragging but oh well.
If I wanted a product that some company offered over what I made, I would have just bought it. But I really like the tunnel extension I make and it is super simple to do and works great.
Basically you just gotta remove the stock tunnel "cap" at the rear of the tunnel. This requires a cutting wheel to cut away the "mushrooms" on the inside from the riveting. Once you remove the cap you can get the thickness of the aluminum and go buy a sheet of the same. Then make a tunnel cap of your own that is the exact same width only 7.5 inches longer.
The difficult thing about a tunnel extension is the "closeout" at the rear that boxes it in. I hate that part of tunnels anyways. So all I do is get a 1" angle of aluminum and cut it to the width of the inside of the tunnel. Then call up Wahl Bros and order up a 3' length of snowflap material from them, it's like $8 a foot. Then I pre-drilled the angle and the tunnel at the rear. Then basically in the end the snowflap gets riveted inbetween the tunnel extension and the aluminum angle. Because it is only an inch it isn't even noticeable. If and when you get stuck just make sure to mount the grab bar a bit above the top of the tunnel so you can grab it still when you buried. The best part of not having a closeout that goes 2-4" downward is it isn't dragging at the times you do get stuck.
I dunno, I've done this exact type of extension to at least 8 Polaris sleds and they all have turned out and worked great. Low cost, great function and not tough to make yourself.