I made a kestrel like clone with a plastic buckle that are weight rated to 500 pounds of force. I'm sure the metal buckles are rated much higher and are most likely safer though. I didn't use leg buckles though and went with the G hooks.
https://milspecmonkey.com/store/har...ra-buckle.html?search_query=Buckle&results=27
I tested it extensively at ground level and could not get myself to fall out of it. I didn't test any force on the buckle other than my weight though. I also bought a mantis and used that extensively. I wanted one to sweat in while prepping trees in the preseason and the other one for hunting. I did hunt out of my diy saddle a few times and it was comfy.
The next one I make though I think I'll use 1 or 1.5 inch tubular webbing instead of 2 inch seatbelt webbing, which was overkill in my opinion. I will also probably use a metal Cobra or similar buckle with it as well for the main buckle.
Overall, I don't know how much weight savings there is between polymer and aluminum. I tried looking for the weights but couldn't find it on Milspecmonkey website.
Unless you trust your sewing ability I would just probably keep the buckles as they are. You could change out the leg buckles if you wanted though.
Just think safety first and carefully think about any modifications you make and test as ground level first.
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