Since I only need to support three areas of the light tent I had plenty of cardboard cutouts left. In my case the size of my box was 12x12 so that left me with an approximately 10x10 square. (I've never, ever been able to cut a straight line..... even with a ruler.) In the close up picture you can see that I have pinned a swatch to the cardboard to help set the shape. Obviously this isn't going to work for really large items but it can work. I've seen some really big boxes around. I also don't like to soak my material because it takes longer to dry. Instead I choose to use a spray bottle and get the object signifantly damp on both sides. Then I shape the item and pin it to the cardboard, using the pins to get just a slight tension so the items doesnt pull back up. That may not be the way to block "officially" but its my way.
I don't know how long I will swatch yarn. I really am much more the run with whatever I have type knitter/crocheter/spinner/dyer. However, I get left over yarn all the time so I'm sort of brewing in my head what I should do with them. I'm sure it would make a great piecemill blanket or something. Until then I am very happy to know I have a quick and easy way to "perfect" my sample knits.
(A Disclaimer - this swatch was already blocked but the previous photos weren't great so I "staged" these photos.)