I have a 1999 and 1998 Sportsman 500's. Not designed for 2 riders (2 up). I have also ridden extensively on a friends Sprotsman 500 2up.
On the 2010, which is most similar to yours, we used this:
J R Graham Powersports
It worked VERY well although, it was a little too far forward for our liking. I am 250 and wife probably 170's both tall. It was not uncomfortable at all, but a little more space front to back would have been an improvement.
On my older machines I bought these:
Sporting Cases And Gun Cases | Wayfair
I had to modify them a little bit, but they were much better comfort wise and allowed a little more space. Modifications:
Built a box out of 1/2" plywood to fit in rear bag and bolted it through the rear rack, which gave the back rest stability. Worked GREAT. Also bought Sterilite "Shoe Box" size plastic contaiers for the side bags. Made them more rigid and really made the ride great.
On both machines we did some very aggressive rock climbs, mud holes and 200+ miles of trail riding without issue.
Your machine is not designated for 2 riders (2 UP), so keep that in mind, but your 400 is not that different from the 500.
Hopefully my experience helps you. Feel free to ask more about it, almost all my riding is 2up