Back in the states they made foam "closure strips" that matched the profile of the corrugated panel, Every place that sold the corrugated panels sold closure strips, this made for an air tight and weatherproof seal. Are you sure they do not have something similar here? The foam strip were about $1 each and very effective, and would make a better seal than the wood.
I was a general contractor for 35 years back in the States and Keith's statement about the Chlorine is correct. I hadn't heard about the drain cleaner but that also makes sense to me. One thing that all my septic installers recommended was the addition of Rid-x or some other septic bacteria on a regular basis to make sure you maintain a health septic tank. http://www.rid-x.com/