I bought two of these Oupost cargo cages to use on my gravel rig and hardtail that I use for bikepacking. While they're not as light as some other cages out there, they're sturdier which for me means more than saving a little weight. The cages are metal. I have other cargo cages that are a composite material but wouldn't last with some of the rougher conditions I've used these in.Yes, there are only two sets of three mounting holes. You can still mount them to a fork though that has bosses. I like the fact that they can be adjusted by choosing any one of the different mounting holes to keep them clear of chainrings, etc..They can carry large Nalgene bottles, Klean Kanteens, cargo bags and even a crowler can of your favorite local beer (see picture). I'm purchasing two more cages to mount permanently to the gravel rig so I don't have to swap back and forth anymore. Blackburn did a good job with these although they could do with a third set of mounting holes. I read a lot of people drill a third hole for fork mounting which could save people the time and effort.