Never had any issue with asking for, and getting, my food put into a take away box and I've certainly never been charged for it. Some restaurants are proactive in offering but if they don't, just ask when they come to clear the table. A couple of places have brought over the packaging for me to do it myself but most do it for you.

Not every restaurant is set up to supply take away boxes but any establishment that offers a takeaway service will do it.