Be careful with the kind of plastic you use.....or with your belt after you make it.
A fella stopped by to ask if i could fix a belt with an insert in it. He had sat on it while he was getting ready to get dressed and folded the belt in half. It made a crease in the insert that will not straighten out. Even while he wears the belt there is a very noticeable crease in the belt.....the insert may have been Kydex but he didn't know.
Also, after 6 months, the belt had not conformed to his body....you know, like that "S" curve we all get in ours.....so according to him it had not been broken in yet and was not very comfortable.