I've used a Budge Tyvek van cover size V2 on my 16' Casita
with really good success and got a good seven years out of the cover About $95. (It covered the trailer very nicely with about a foot showing all around the bottom)I now own a covergiand cover which is still very good but the Tyvek is much lighter and durable that i may go back and get another. I don't know the exact measurements of your trailer but a quick call to the company or click on the link for sizing would help if you want to go that route.....
Theres lots of great covers out there but this lightweight very durable cover worked for me and the price is right.
Be sure to size it out in the "V" Series (van)
I just found the Exact Sizing Chart..... V2 is what i used: