The Seascape 18 has a strong weather helm and it's not really possible to leave the tiller unattended without some kind of locking mechanism. It's also something very useful to have when sailing alone. There are a few different designs out there like Sven Orlowski's "auto pilot" which this implementation is based on or let's say Thomas Nielsen's "2 second self steering system & battlestick velcro lock".
I was hesitant to make permanent changes to my Seascape 18 like drilling holes into the tiller tubes (Sven's design), or glue things to the hull that are likely not UV resistant and possibly a mess to cleanup (although I like the Velcro idea).
Here's my interpretation of a clean, reversible, non-destructive and no-mess tiller lock. Instead of the Nylon square rod I'd maybe recommend to use some other material that is a bit easier to work with. Maybe wood?