JW Pet Walk Up Barn Small Animal Toy is the ultimate small animal accessory.

Cool, translucent small pet hideout gives your pet a place to rest & exercise in one!
Hideout features ramp steps on one side and a spacious entry door on the other.
Ramp steps make it easy for your small pet to exercise and climb to the top of the hideout to lounge or rest.
Opaque plastic construction allows you to view your small pet as he cuddles and nests inside. Assorted colors & sizes. Made in the USA

Review
Large Size measures 13-1/4" x 9-1/2" x 7" high
Solid with NO ventilation holes.
Steps are slippery, rats with weak hind legs may have problems trying to climb it, but rats without physical handicaps should have no problems.
The top is large enough for a tiny/small fleece bed to be place.
I used self stick velcro on barn & sewed a strip on the bed. (seen is one from Petco)
My rats sleep both inside it as well as on top of it. No chewing on it (yet). Inexpensive.

Recommendation: My only concern is lack of ventilation holes, but without the holes there is no possibility a rat on top can eliminate on the ones inside.
Would recommend it for healthy rats.