You could hire a professional to build you a cat enclosure or buy a kit that you can put together yourself. These videos offer you some simple do it yourself solutions.
Turn an IKEA bookcase into a window catio.
Attach mesh around an apartment patio.
Attach chicken wire and garden netting around a balcony.
An inexpensive cat enclosure built against the house using the window for easy access by your cat.
Another cat enclosure built against the house using the window for cat access but with additional features such as carpeted perches and a scratching post.