The simplest and cheapest hack that I can think of is a simple elastic band. Loop the elastic band over the inner tube valve of the front wheel, stretch it over the downtube and back over the inner tube valve from the other side. This will immobilize the bike and not allow the handlebars to turn. You can then lean the bike against a wall without worry. I have a bowl full of elastic bands in my kitchen that were wrapped around mail that I received, an unlimited free supply