If its a screen top your local pet store will probably sell the clamps for it but I find them to be quite flimsy and the snake can usually lift the lid enough to get out. I bought a whole new screen lid that came with clamps that hold it down nice and tight.

On most of my other cages I just use heavy books or rocks.
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0.0.1 Green Water Dragon
1.0.0 Tokay Gecko
1.1.1 Bearded Dragons
0.1.0 Columbian Red-tailed Boa
1.0.0 Yellow Ratsnake
0.0.1 California Kingsnake
0.0.1 Black and White Tegu
0.0.1 Chilean Rose Hair Tarantula
1.0.0 Evil Budgie