The wood for the real windows is Meranti too. The same kind of preshaped profiles, which are too large for my little shed. So I had to rip and plane them (all electric!) to a smaller size. For cutting things to length I don't have an electtic chopsaw, I usually do that by hand. Now I remembered the old Ulmia mitrebox, restored last year. It's a great "machine". Pretty fast and very acurate.
Some time ago I bought a couple of typical Dutch mortise chisels. They are huge! The blade tapers in all directions, in the width, thickness and length. The handle is ton-shaped. In fact they look a lot like the ancient chisels in Roubo from the 17th century. The Dutch, and I think the Germans too, just kept on making them like that. The largest one is a nice size for the windows, so I sharpened it up.
You can see two styles in this picture. Both are sold as Meranti, but one is quite soft and the other is hard and britle. I've heard that Meranti is one name for many species of wood from Asia. The two are totally different. The softer one is very easy to mortise. The other one is more work.
Her you can see my setup. Style clamped to the bench. An extra clamp on the end to prevent spliting and the chiseling spot above a bench leg.
And the result, 4 mortises for one window. The chisel works as advertised.