I'm thinking maybe wedi board. Not lots of flex and very light. Or, you could consider using metal lathe cut to your size and a coating of cement to fill it in for the mosaics to adhere to ? I do think it is doeable also, but will be darn heavy .

definitely the Wedi board is what you need

Here is a link that might be useful: wedi at maryland mosaics

What ever happened to the people who were experimenting with a thin layer of mortor on styrene or was it blue board? (Cheaper than wedi board)

Otherwise wedi is the way to go.

Kathy would you consider making it in several pieces, like a triptych?

Kathy

I was just thinking of making it in 3 pcs ! You read my mind ! It'll be alot easier on the walls. I'll have to look up Wedi board - It's been so long - I forgot about it . I have some homasote here , but I keep it to solder on. Off to check out wedi !

that was me concreteprimrose, it works but its heavy. ChickchickchicKENs ended up weighing over 40 lbs

Wedi board is the way to go. I have ordered it in the past from Maryland Mosaics. It must be becoming more well-known, because now a local tile company carries it (Daltile) and they let me buy some from them even though they sell only to contractors, they make an exception for mosaic artists. After I went there and picked up the Wedi, I went to my local stained glass shop to place a large order for glass, and saw that he now carries Wedi also! Yay! And he carries MAC glue now. Anyway, you will be amazed at how light-weight Wedi is. Also, Wedi has a special hanging system you can buy. I recently found out that Michaels (our local arts and crafts shop) now carries a French cleat for hanging heavy objects. Just Google it and you will see what I'm talking about; it's a really clever and simple way to hang stuff.