[Walmart] 1" Memory Foam Mattress Topper (Twin/Full/Queen) $19.88
Come with 3 sizes for Twin, Full and Queen Mattress; Not sure about King, since I can't find it.
This is topper so it's about 1-2 inches smaller than the mattress!
If you want an exact fit for a twin mattress, You should buy the Queen size and then cut it
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