if you use single thickness computer printer paper you will have to do this again fairly soon.
You can use cardstock but it is harder to work with and can be frustrating.
Personally for this example i grabbed an envelope
it is basically double thick computer paper but it made a quick job of it.
Measure the paper to size of the spine.
folding it into the spine you will get the correct thickness as well as length.
FOLD the paper in thirds and GLUE it.
you want to end up with a flattened STRAW
if you find a paper straw the right size you are in luck.