This 34.9mm braze-on adapter weighs only 20g, saving up to 10g over a Shimano adapter. Tuned with a titanium bolt, it comes in around 18g. Expect a 31.8mm to come in a gram or so less. They come in black or silver, and are resold under various brands, but this one had no branding. Note that these have a flat front and do not require a curved washer, but that also makes it incompatible with the SRAM chain catcher. If you’re building a bike up from scratch, there’s no real reason to avoid this. At $3 and saving 3.3g per dollar, it represents a very good value, very low cost upgrade. It can be found on eBay.
Price: $3 [Very Low Cost]
Value: 3.3g/$ ($0.30/g) [Very Good Value]