[... catch { console show } ...]
This gives you a toplevel where stdout and stderr go, and you can also interact with the Tcl interpreter.
The menu page has a piece of code that you can use to add a 'Show Console' operation to the system menu. It's a handy thing to have in any Tk application.
RS very often adds this little debugging helper to a Tk script:
bind . ? {console show}One could use <F1> in place of ?, if the toplevel takes keyboard input.