Don't scoff too much at the idea of cardboard. There are plenty of pretty archival cardboard materials that are less expensive than matt board. Take a look at the same material that archival boxes are made of at University Products for example. I would have said Light Impressions, but they are horrible now.
Well this wasn't an archival cardboard. It was acctually the cardboard that Ilford uses in their 11x14 paper envelopes to keep the paper from getting bent.