Hello; I found the last plastic corner protectors at a local hardware store. This is one of the surviving stores that has been in business for many decades and had the good fortune to be far enough away from the new big box home improvement stores like Lowes or Home Depot to be able to stay in business. I have checked in Lowes, Home Depot, ACE, 84 Lumber and the like with no success.
Ask for protectors for the outside edges for wallpaper. They were a bit too wide so I carefully scored them with a utility knife while having them clamped securely to a work table with a four foot straight edge. I took about a 1/4 inch off each side so that they would not extend too much. On bigger tanks with thicker glass and for the purpose of adding strength no triming would be a better way to go.
If you want more strength and do not mind an opaque material, perhaps thin angle aluminum would do. I suppose it could be painted black to more match the plastic trim.
