You won't find a kit. At least I'm not familiar with one.
Your question is a little vague & I don't know what level of effort you are willing to exert. Don't know what type of wood craftsman you are?
First, pick the kind of wood you would like to use. Oak, Maple, Cedar, Redwood, or something else that is pretty weater resistant. I guess you could use pine?
Next, depending on how much work you want to do you could: * Cut and mill pieces of wood to fit inside and around the edges. You probably want 1/4 inch thickness maximum. This could produce a very nice custom look. Put some spar varnish on the wood for protection, and glue or nail in place. * Or, you could find some prefinished wood like paneling and cut piceces to fit inside top and sides. Then find some decorative trim to go around the edges. Glue or nail in place. Won't be as nice as first option but could be satisfactory.
I think you should spend some time looking around in your local hardware superstore (Lowes, Home Depot, 48 Lumber, etc.) or lumber yard and see what they have that might work. Make some measurements at home first, and take a tape measure with you. Then look at the wood products they have to offer and ask yourself how they might look. These places make hundreds of types of trim picecs. One of them may be your answer. If you go with the pre-cut pine trim pieces, you can stain ant color you like and protect with spar varnish also.
Good luck and go out and be creative. You may want to buy enough and experiment with one window first.
BTY I hate Lowes & Home Depot. Oh well, monopolies will prevale...