Lux element, alternatives?

[webkocken]

We are going to enclose our wall-hung toilets, i.e., the actual fixture. There needs to be a waterproofing layer behind and under, and then we plan to enclose everything in a box. Lux elements (construction boards) are an option. The question is whether there is a cheaper alternative to those boards?

 

[Onsalakorven]

We are going to enclose our wall-mounted toilets, meaning the fixture itself.
There needs to be waterproofing behind and under, and then we plan to enclose everything in a box.
In that case, lux elements (construction boards) are an option. The question is whether there is a cheaper alternative to those boards?

Raising this question, same situation! What is the best alternative?