eBay UK has lots of Durst parts as these enlargers were sold extensively here. Item 221173456625 is a 6x6 insert ending 13 Jan and sitting at 99pence (2 USD?). 221173403111 is a pair of 35mm inserts at 5.50UKP (10 USD?) ends 13Jan. I don't see other inserts at the moment, but they appear quite regularly. Your carrier is the same as mine, but different colour. Inserts or glass sit in and are held by sliding catch. Glass needs a bevel all round to fit.