I use the medium (1 1/4 inch) black binder clips from walmart. It's only a little more than a dollar for 12 of them, and they do work really well. With my first tub I used the large ones, but they end up sticking out too much from the tub for my taste, plus you only get 6 of them per box.

I thought about using luggage straps/bungee cords, but I'm so paranoid about my snakes escaping and, at least in my mind, they wouldn't be as effective as the clips that actually pinch the lid and the side of the tub together. But I have heard that some people use them with no problem, so it's up to you.

So yeah, the clips work well, but like I said, I'm getting sick of having to take them off every time I want to handle my snake, and then putting them back on. I'm sure it would be much easier to just be able to slide them out like an actual rack. I'm thinking of trying to use rabernet's "Frugal Herper's Snake Rack" sticky, and put some eucaboard (or some kind of board) on the underside of the shelves. I actually thought about getting that plastic unit that she shows in the sticky, but I really like how sturdy these metal ones are (plus I think they look nicer haha).