When you're trying to save money and the environment at the same time, making a futon frame from reclaimed barn wood or old boards sounds like an easy project. But the mattress to turn that frame into a bed gets more complicated. You need to stuff a fabric envelope and then cover it with a removable, washable cover and ensure the whole thing is both firm and supportive, and thick enough to provide cushioning. Expect to spend some money when you buy new high-quality fill. But expect to save, compared to the cost of a store-bought futon, when you make a traditional cotton mattress yourself.
Determine the size of your mattress -- stick to a standard size unless you are customizing for an unusual space. Twin futon mattresses measure 39-by-75 inches; full are 54-by-75 inches; queen are 60-by-80 inches; and king measure 78-by-80 inches. Futon love seats and chairs require different mattress sizes.
Lay out sheets of cotton batting in the size that corresponds to the size of your mattress. Stack the batting 4, 6 or 8 inches high; cotton batting doesn't compress as much as foam fillers, but a higher mattress might work better for a heavier weight.
Tuft the stacked layers of cotton with an upholstery needle and sturdy thread to keep the batting from shifting during use. Draw the thread through all the layers about every 6 to 9 inches and knot it off underneath. For a very high mattress with many layers of batting, tuft a few layers at a time and tack adjacent layers together at the corners.
Stitch a fabric envelope to contain the batting from muslin or a couple of cotton sheets. Sew two rectangles of fabric, right-sides together, on three sides, double-stitching at the corners. Turn the envelope right-side out. Fold under the two raw edges of the open end and hem.
Stuff the batting into the envelope, pushing it into the corners and smoothing it out. Blind-stitch the envelope closed at the open end. This cover stays on the filling permanently. The outer cover is the one you remove for washing.
Make a fancier, removable cover with a zipper, button or tie closure that will slip over the futon mattress and is washable.