The term "well-constructed" implies something that is strong, sturdy, and built with care. When looking for antonyms, words such as unstable, poorly built, or fragile come to mind. An unstable structure is often wobbly or likely to fall apart, while a poorly built item may have been constructed hastily or without attention to detail. Fragile suggests something that is delicate, easily broken or damaged. Additionally, flimsy, shoddy, and inferior can also be used to describe something that is the opposite of well-constructed. These words all indicate that something is lacking strength, durability, or quality, and therefore cannot be considered well-constructed.