My Adco cover that I use on my 210WBS has 4 tabs, front and rear, that you can run a rope through to take up the slack...should be enough adjustment to pull it snug. Also has straps that run under the trailer to keep the edges taught... You'll have some gathers on the roof, but, the cover should be snug enough for you to use.
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)