Just an update if anyone in the future wanted to go this route for printing 6x9 on LPL machines:

Shortly after these posts I cobbled together a mat board carrier out of some black mat board. As I suspected the negatives could not sit flat at all, and when the mat carrier was inserted and locked down (the LPL 670XL has a spring tensioned locking negative carrier slot) the locking mechanism bent the paper board. There was also significant light leakage around the sides and bottom.

The solution to this was to take apart another negative carrier and use its bottom portion as a stable base and guide for the enlarger head. I therefore took apart a 6x6 carrier, only 4 screws, two are secured with small nuts (dont lose these) and two metal clips that hold the opening top portion. The bottom of the carrier is plastic and the base is molded to fit the enlarger head securely and also aid as a guide when inserting.

I took a glass sheet and cut out two pieces of glass, and taped the whole thing up together. Mat board with 6x9 openings, and two pieces of glass on the inside. This was taped onto the bottom portion of the 6x6 negative carrier. It worked very well! It was stiff enough and stable when inserted and locked down, and the glass kept the negatives flat, also the original base prevented light leaks.

Yesterday I finally received another universal glass negative carrier in the mail and proceeded to take it apart. The bottom opens up easily with the removal of 4 screws, and the masking vanes are then accessible. I tried thinking about how they were assembled and doing the reverse to remove them but they were locked in pretty well. The yellow tabs that control the vanes lock them in, the metal vanes were inserted into them on assembly, but I couldn't pry them out. The only way to get them out was to bend the vanes slightly to remove them. The metal bends easily so you wanna go slow and not over do it. So just pull up on the vanes and you can wiggle them out. between the vanes and the carrier is a small piece of spring steel that tensions the masking vane to the carrier, you can see it in one of my attached images, dont lose these as these keep the blades in place and not sliding around in the carrier. And after replacing the bottom, you have a 6x7 carrier that fits 6x9 with a bit of the rebate showing on the edges.

The attached images show my initial mat board and glass carrier as well as the altered universal carrier.

I have been getting away with printing the images on an 80mm f4 componon and a 60mm WA rodagon. With the 80 I lose a tiny bit on the sides, with the WA it goes right to the edge, but I cant print the side rebates only the top and bottom. I'll have to spring for a 100 or 105mm lens sometime, but as of recent the prices for them were a bit high. A 80mm WA would be awesome as well. (Ive been using the WA lenses a lot 40mm and 80mm and they are nice to print larger with)