What you need:
Square section tin spouting
Untreated wood about 20mm thick
Step 1: Materials
After a look thru my wood pile I found some untreated 150x150x20mm timber for the ends of my window box.
Step 2: Assembly
Mark and cut two pieces of your untreated wood so they fit snugly in the ends of the spouting.
Pre-drill the spouting about 10mm in from the edge and put a bead of sealant along the edge. The sealant is simply to stop water leaking out around the ends onto my window sills.
Use wood screws to secure the ends of your window box on.
I used four self adhesive rubber feet on the bottom of the window box so the screws on the bottom dont scratch the window sill.
Step 3: All Done