A hard shell trombone case can be very expensive, especially after paying a great deal for your trombone. You can bypass the cost, however, by making your own soft shell case, essentially a gig bag for the instrument. When complete all you need to do is add a strap and it can easily be carried over the shoulder, leaving the hands free.
Step 1 - Canvas
Start by removing the slide from the trombone and measuring its length. After that measure the width of the bell of the trombone plus the best of the instrument, and of the bell alone.
Cut 2 pieces of canvas, each of which should be 1 inch longer than the slide and 1 inch wider than the total width of the trombone including the bell. These will form the top and bottom of the trombone case. Now cut another piece of canvas that’s 1 inch longer than a measurement taken all around the edges of one of the other pieces and 1 inch wider than the trombone bell.
Step 2 - Sewing
Using the sewing machine, sew one side of the thinner, longer strip around the edge of one of the other pieces of canvas. This will give you the bottom of the trombone case and the sides. You should have a small overlap of materials as you finish the sides, but sew this down. Use heavy duty thread for everything and test the seams to ensure they’re strong and won’t come apart.
Step 3 - Lining
Cut a piece of foam rubber that will fit inside the case and cover the base. Glue it in place. After this, cut another piece of foam rubber that will cover the sides, but leave 1/2 inch of canvas at the top. Glue this in place and allow to dry. To complete the lining you’ll need to cut velvet to fit over the foam rubber. The simplest method is to cut first for the base, then the sides and glue in place. The velvet will stop your instrument from being scratched by the foam rubber.
Step 4 - Lid
The lid of the trombone case will perform a double duty. First of all it will complete the case, but it will also hold the slide. Cut foam rubber so it’s 1/2 inch smaller than the case on all sides. Glue it in place.
Now cut two pieces of velvet that will cover the foam rubber. For one of them, fold three of the sides so you have 1/2 inch of material folded over. Sew this to the other piece of velvet on the folded sides, so that you have a pocket to hold the slide. Glue the velvet in place over the foam rubber, with the slide able to move in and out of the pocket.
Step 5 - Final Assembly
Place the lid on the trombone case, velvet side inward. Sew one of the long sides of the lid to the side of the case. After this, sew a zipper in place on the other 3 sides and check it opens and closes properly.