I have two recommendations for you.

The first is luggage straps. Many people use these to secure their tubs.

The second is clamps. Go to Lowes or HomeDepot and look in the hand tool area. I have and have used small clamps I bought from there that were $2 each. They are really strong but easy to use. I used them to secure the lid of a tub holding my adult Blood while I waited for her pvc cage. It ended up taking much longer than expected so I used the tub and clips for a few months with no issues.