The next time you have to secure a couple of boards together but you don't have a clamp handy, the solution may not be in your tool cabinet but in your medicine cabinet.
It's amazing what you can do with an elastic bandage. Place the two boards together and begin wrapping. It is important to stretch the bandage out as far as you can each time you go around the boards. This is where the pressure comes from.
Simply tuck the end of it underneath…and you should have all the pressure you need.