Another small thing is that a matte box is great to make people know that you are filming with a DSLR. Even though you might have rig, LED light or mic on top of your DSLR, because of the form of a DSLR allot of people still think that you are taking a picture and using a matte box you can hide the DSLR behind it so people don't mistake it.
$15 - Matte Box
$2 - Cokin 82mm adapter
$1 - Cokin P filter holder
$2 - Cokin hood
$15 - Cokin P194 Gelatine holder
$20 - Cokin Set with ND filters
Another tip for when the flags of the Matte box start to get loose and not hold it place is to put super glue in between the joints and keep moving the flags until its fully dry. What will happen is that the glue will seep into the cracks and close the open space and give more grip to keep the flag in place.