Once you've gathered your wood choose the straightest and cleanest piece for your top dowel.
From the measurements taken earlier:
|window opening:||103cm (40.5")|
|allowance on each side:
|top dowel length:||123cm (48.5")|
This design calls for ladder rung width of 30.5cm (12") in length. Based on my calculations I figured 12 rungs would put me close to the ground. Cut each dowel to length.
When finished, you should have one long dowel and several shorter, equal length dowels for your rungs. Gather up your goodies and head on back home to finish fabrication.