Perspex -one of the MethylMethacrylate polymers used for windows and aircraft canopies.
The relevance is that if you can describe your Bucket Cell Adaptor as an art piece rather than an engineering piece someone who makes and engraves trophies will have the tools to make your bucket cell adaptor plate out of your chosen plastic -PVC or cPVC in this case. Let's face it, few of us design a cell to mils.
Please be cautious with big cells. The conditions to make chlorate by electrolysis are favoured by warm conditions (ask Mumbles for the precise thermodynamics!) to get and keep a IBC size cell hot would take much too much current for a domestic supply and make a commercial amount of product and waste. One of the significant design elements of a cell is suiting the current to the cell size, current not used for electrolysis will create heat, that heat is lost through the cell walls and heat produced will match heat lost at a certain temperature rise above ambient.
Probably a reasonable top limit for a "home" cell is the amount of damage that a container failure would make -and what is acceptable. IMO the best uses for an IBC would be holding water at a site, and using one as a tray in which to contain the whole wet side of the process. Anyone doubting the risks associated with too big a plant should search youtube for "pepcon" or just consider the herbicidal effect of chlorate over a radius of 500ft if your cell leaks into the ground.