[KellyV]

They make tops/covers for xpens....
Exercise Pens and Top accessories, Midwest Exercise pen, Midwest Accessories

Midwest Exercise Pen Top - Houses & Outdoor Kennels - Dog - PetSmart


I'd probably also use tent stakes or something to tack it down into the grass when outside just so it doesn't accidentally tip over or something.
Stakes for MIDWEST Exercise Pens - Houses & Outdoor Kennels - Dog - PetSmart

Otherwise you could get something larger for more permanent type use.
PetSafe Outdoor Chainlink Kennels - Houses & Outdoor Kennels - Dog - PetSmart