i really don't want this to be a catastrophe and fall over during the reception.
I think this is a point to make to the customer, that you are providing something that will be safe in an environment often associated with children running amok, drunk people, and raging bridezillas, and in order for it to be safe, it will be susceptible to falling down if struck, run into, tackled, or kicked over. This sign falling over won't be the end of the world, just another fun memory for the couple, when little cousin jeremy decided to show off his flying tackle on the display (I'm imagining cousin jeremy as a 30+ drunkard, but of short stature)
If they want something that won't fall, you'll just need to figure some wall mount or something more heavy duty, which is just going to add cost for something temporary.