Make them yourself!
1/2" or 12 mm aluminum is ideal, preferrably flat bar or a side of an angle, as extruded profiles are generally stronger than plate material. 6060 or 6082 aluminum will be fine for this purpose.
Then use a center punch to mark the centers of the large hole and the 2 bolt holes. Then draw the outside lines with a pair of compasses, to your required shape and size.
(or just use the OEM flange as a template, and scribe around it)
Then drill out the large hole with a hole saw, to fit the required header diameter, and the 2 bolt holes with a regular drill bit, 10.50 mm is fine. Then use a jigsaw or bandsaw to cut the outside shape, then file and sand to the required finish. Your new flanges will be a lot lighter than stock, I would guess around 270 grams or 9.5 oz.
If your pipes require a "neck" on the inside of the flange, a piece of aluminum or SS pipe of the required size will do the trick, just slide it on before the flange.
When cutting aluminum, I use CRC Supercut, or a similar cutting oil, to avoid material sticking to hole saws, drill bits and saw blades.
Good luck!
