Without writing oodles of code that is not easy. What is easy is to use the Status Bar (bottom left of screen). This is the area that Excel itself uses. To put a message on the Status Bar use something like:
Application.StatusBar = "Working ... Please Wait!"
when finished use
Application.StatusBar = False
(Do not be tempted to turn off your message using Application.StatusBar = "" because you need to free up the bar for Excel to use later on.)