Thin, springy strip brass is available from any modelmakers shop or tool stockist and aluminium or stainless-steel is also widely available. Just buy 25mm wide, drill two holes, bend it in a vice and you're finished. There is no precision required. If the screws are also missing then take the part along to the shop to make sure that you get the right size -- a Durst will probably use M or MF threads while a US made enlarger is more likely to be UNF or something like that.