Google "ladder safety device", loads of products out there, not to mention getting someone (if possible) to properly foot the ladder. By screwing down a piece of wood your effectively putting your faith into a couple of screws attached to decking of which you have no idea regards condition. Mad idea. Putting your ladder up against something solid, yes, a Heath Robinson knock up job of a saftety device, no thanks. Back in day, I used to use ladder mats and they were brilliant. If your gonna use ladders, do it properly.
I have fitted those small rubber rectangle feet on all my ladders, I think they cost £40 a pair. Drilled a hole in them and now all I do is put a 3 inch screw through the hole and into the decking. I don't tell anyone and go ahead and do it, my safety comes first. If its the man made stuff then you could look for the joist and screw between the decking boards so you miss them, I guess.
Last week after the storm I was cleaning a new detached home and the downpipe had came away from the running outlet. I took a photo of it, put the ladder up, fitted it back together and drilled in a small screw. Put another card through the customer's door asking for a tenner for fixing the downpipe. The guy opened the door thanked me and put £15 in my bank account.
Even if he refused to pay I wouldn't have cared as it's a great clean every month and would have done it for nothing as the tools are always in the van.
Saturday, I showed a customer their slates came off in the storm and are sitting in the gutter. Put ladder up and brought down the slates and left them under the front window. Checked the other side of roof where a few other slates were off and reported all to my customer. I have the job of repairing the roof and clearing out the gutters for nearly 2 ton this week. If they claimed the factor or insurance it would be much more because of the excess. I run my business like this, if I see something wrong I usually fix it if I have the time. It seems to work and I don't lose many customers because they know I won't do any other work for them.
Every window cleaner could adopt this service and the money flows in, this is what I have noticed. fwiw