Use a program to design the front panel overlay - I use Adobe Illustrator, but that is not the easiest to use (or the cheapest). Something like CorelDraw is more user friendly, but even Word, Publisher or even Powerpoint can work just as well.
Measure the 'recess' area of your box, and make your design a few millimetres (1/8") larger all-around - this is called a 'bleed', and is there so you can chop off the edges and have color going all the way to the edge of your design. Work out where you want any buttons or other components, and accurately position text to label these. Choose an interesting font (preferably other than Times New Roman and Arial - these look boring just because they are used for everything!)
To make it look really classy, put an image behind the text - I use a istockphoto
which costs only a dollar or two for that perfect background image, but you can easily find free ones on a Google Image Search
as long as you are not going to churn out thousands of widgets to sell and get yourself in trouble with licensing issues.
Finally reverse the design (most graphics programs have a "mirror image" function), and print it out on an OHP transparency sheet. If you can't find out how to reverse the image in your package, you can probably tell your printer to do this for you (many inkjets have this function built in to allow you to do things like T-Shirt transfers). Failing that, you don't really even need to reverse the image - but if you can, it just means you can turn the transparency over after printing, and the ink will be on the reverse side of the plastic, and will then stay protected for life and not scratch off. Alternatively, you could use a 'fixative' spray on the ink (like a lacquer), or stick another clear sheet of OHP film on top. Here is an image I have printed reversed from my color laser (the photo here shows it turned back around the right way). If you are using a laser, make sure you get a proper 'laser/photocopier' transparency that will withstand the heat, and if using an inkjet, use a proper inkjet transparency that is coated to make the ink stick.