This 34.9mm seat clamp weighs only 20g, although I have had some that weighed 22g. Expect a 31.8mm to come in a gram or so less. They come in several colors, and are resold under various brands, but this one had no branding. These have a lip, an angled slot, and a threaded steel barrel for durability. Some stock seat clamps weigh upwards of 30g, but I don’t have any on hand. If you’re building a bike up from scratch, there’s no real reason to avoid this. At $2 it represents a very good value, very low cost part. It can be found on eBay.
Price: $2 [Very Low Cost]