Many years ago, Ilford Australia had a trade presentation regarding this problem with Cibachrome.

Their presentation, was the first time I had seen foam core used as a photographic base.

Since then, I have used foam core based products on and off with plastic papers and to my knowledge I haven't had a print suffer.

For large prints I used Ademco 19mm thick board, made in the UK, they were the only one I knew of in the eighties. These days there are quite a few suppliers of this kind of product and the web makes finding suppliers easy.

Using thick mount board gave us an idea of making a life size print of a person holding a shining torch. We made the life size print, then cut out the image of the torch beam and replaced it with a backlit Duratrans. The client loved it, the only problem was, it was too expensive to produce for the numbers required by the client.