MDF was also my choice for my largest gluing project. I needed about 7m² for the 24.000 Educa. MDF is cheap and easily available in big sizes. I'd use Plywood f

MDF was also my choice for my largest gluing project. I needed about 7m² for the 24.000 Educa. MDF is cheap and easily available in big sizes. I'd use Plywood for things <1m², it looks nice. For the 9.000s I always used paper for the backside (old calendars are a good source for big sheets with excellent quality). Using one wooden lath at the top was enough to ensure stability for a 18.000. The 9.000s didn't need that - just paper and a bit of duct-tape on the backside were enough.