I bought one of these and at first I thought it was to big but it will fit. It is a tight fit but it does go in. Need to push it towards the rear of the truck. Also seems to have loosened up a bit since I put it in a couple of weeks ago.
Truxedo Lo Pro
Had one on my 16 and currently have an X15 on my 18 but will be going back to a Lo Pro for the 21. The snow does not slide off of the X15 canvas very well and sometimes freezes on there.