To sew your own canvas belt you need offwhite canvas, offwhite thread, sewing machine, pins, iron.
First you cut the fabric into two long pieces. They should be about the same width as you ammo belt piece. The belt will turn out a bit narrower because of the seams. The belt shoud be long enough to fit around your waist in armour, with an overlap of about 20 cm.
Sew the two pieces together right side facing right side with a straight seam followed by a zigzag seam to keep the edges from fringing. You can also use an overlock seam as I have done if you have that kind of machine. Leave one end open.
Turn the belt inside out and iron the seams so that the seam is tucked well out on the edges. Pin if you want to.
Make a straight seam 2mm from the edge around the belt. If you want to have some soft plastic support inside your belt, tuck it in before you close the opening by tucking in the fabric and closing with a straight seam. Sew velcro on. If you have plastic you can't sew through, you have to sew the velcro on before you sew the belt.