... I can vouch for the product offered by eBay seller "oldferrari"
http://www.ebay.com/itm/Replacement-Insert-Rolex-GMT-6542-Bakelite-/330662785565?pt=LH_DefaultDomain_0&hash=item4cfd09b21d
I bought one of his first batch thinking I'd squirrel away my genuine bakelite insert.
The overall look and gentle ageing (and certainly the price) of his product is better than many I've seen.