Find a piece of felt twice the size you like and fold it in half. Keep in mind we will be leaving about 3/4" on each side for the bottom floor of the bag. That way it won't be flat and you will actually be able to put stuff in it.
Wow. That felt looks cat-hairy... but we don't have any cats... maybe it's thread? Anyway, moving on. Sew along the folded edge.
Now turn inside out so that the seam is on the inside. See?
Now sew straight seams 3/4" from the first (center) seam making three in total. You now have the front, bottom, and back finished.
Now go through your ungodly large felt scrap bag and pull out some 80's appropriate colors. Turquoise and fuchsia anyone? I also found some weird checker fabric that could work as speakers. Score!
Before sewing the sides on, I like to lay mine flat so it's easier to sew on my embellishments. I used my zig zag on a super tight stitch in order to make it look "embroidered". Using lime green thread really sold the theme. Go around everything. My boom box includes a cassette player (totally boss), speakers, various buttons, and an AM/FM tuner.
Measure the width of the bottom and length of your new sides. Cut out two rectangles from black felt and sew them on either side to your front, bottom, and back.
Now put some handles on that puppy and get jammin'! I just braided together some scrap t-shirt material to make my handles. Good luck