Not only for narrow gauge, but for instant microlayout bases and the like, consider using pre-made Resin Art baseboards available from Riot Art and other arts suppliers. They are made from Beech wood with an MDF support on the back. My lovely wife (who is into resin art) gave me a small circular board and I have purchased a 60cm diameter board, both will be used on microlayouts eventually. While these boards are probably too pricey and restrictive to use on larger layouts, they are ideal for 1 off quick-build projects. Riot art in NSW have a sale on presently, where if you have a loyalty card, 15% comes off the price. There are round ones and rectrangular ones. It could be worth a visit to your local store.
All care, no responsibility I'm afraid. I do not have a hope of controlling my own hobby spending, let alone yours!
