Well the Eucabord could react with humidity and water. While it won't happen with plastic.
The eucabord is there to support the tubs and heat tape. You could use something else, but I don't think you will find something that will do the job as cheap.
btw : Was there any one who know where to find these in Canada ? Even on the US lowes website, I couldn't find them.
The only thing close to it was this
http://images.lowes.com/product/045989/045989017267.jpg
And this is what I find around here in Canada but a bit more expensive.
The 24'' versions are at 89.99$. Not sure it's even worth to build these instead of melamine, other than it's "light weight".