Very important to level the base. We did our best but at the end my hunny says for the effort and headache he wishes he just got bags of cement and poured it. 
The size is exactly what we needed and I’ll be glad to get my garage overflow out. It had a few areas of imperfections and cracks in the plastic inside- but it’ll be covered up with my shelves and honestly wayfair has been so great to deal with when stuff comes damaged I don’t even feel the need to reach out for these little areas.
Glad I got these and I’m very happy with the purchase