Reasonably authentic looking belt buckles can be made using pewter. Traditionally the mould would be carved from soapstone or the model was pressed into clay. The goal of this project was to make a buckle quickly so I (Penda) turned to the materials I had at hand.
We used Plasticine (modelling clay) for the model and Durabond 90 (plaster) for the mould. The pewter came from old pewter mugs we found at yard sales. This allowed us to make the buckles with a very low-cost factor- 50 cents for the pewter and the rest we had sitting around.
The first thing to do is find an authentic source for the buckle. This is very easy to do as buckles are one of the few artifacts that are commonly found. Do a google image search for an Anglo-Saxon buckle, and you will find lots. Many of them will be way too complicated, but if you browse through the images you will find ones that a common person would wear.
Some other pewter castings we have done using RTV silicone: