Josh, during the initial stages of my 65 rebuild I decided to tear apart a heater box just to find out how they were built. At that point in time I was buying parts from all over the country. Long story short I had about 10 boxes. I picked a box at random and drilled out the spot welds. I was amazed at how filthy the inside was. Dirt, leaves, mouse droppings you name it. I took apart a few more hoping to find something cleaner, all the boxes were pretty ugly inside. I do not think that the condition of the box was specific to any geographic location as they were all pretty rough. I cannot recall what year of car that you are working on but I have to think that all the NON AC boxes are pretty much the same. I thoroughly cleaned up some usable parts and replaced the foam as much of the original was worn or missing. I do not recall the source but what you are looking for is a closed cell flexible foam at about a 1 -1.25 lb density. This type of foam is used in the refrigeration business and is similar to the outer foam jacket used on home AC lines. This type of foam is available in various thickness. In my heater box I have two flapper vent doors that required new foam. Plus a large horse shoe shaped gasket. Unfortunately to repair properly the whole box has to come apart, cleaned, re gasketed and welded. I have about 20+ pictures of the tear down and rebuild but is a huge file and not easily sent. After the repair my heater box works very well and the vent doors operate with very little effort at the dash heater controls.