You can make this without a pattern using stretchy fabric. Take a piece and wrap it around you and pin it tight enough so it stays up, then mark where the pins are with more pins, or chalk or one of those markers that disappears with water. It helps to have a friend do this part!
Next, lay the fabric out and tidy up the markings if necessary. Place the two marked lines together, right side of the fabric together, and sew. Take a piece of elastic and hold it around you at the height of the top of the garment, tight enough that it will stay, and cut a piece that long. Sew the elastic to the top using zigzag stitches or a serger, fold it over and hem. Hem the bottom at your desired height.
You may want to add straps if the dress or top wants to scooch down. Cut some ribbon or more of your same fabric and put it over your shoulders while wearing the tube to find the right length.
Strapless tops & dresses usually have a built-in corset that the dress is attached to, which is a much more complicated thing. Since you said "tube", though, that implies a much less structured garment, as I have described.
Post pictures if you make one!
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