I use two clear plastic tubs connected together with a pipe. They both came with locking lids that fit fairly tightly over the enclosure, and have clamps that lock them into place.

When I was more paranoid about escape attempts, I used cow clips on the sides as further security. But these days, I don't worry about it. I've never had a snake escape from being able to slide between the enclosure and the lid. But I also have foam insulation tape between the lip of the lid and the lip of the enclosure, that further tightens any gaps a sneaky snake might try to squeeze through.

Honestly, even with as much space as there is in the enclosure, I get the feeling that Little Dude gets cramped in there. I've started taking her on "walks" lately - that is, taking her outside and letting her crawl around in the yard for about 15 minutes a day. Some days, she'll explore, explore, explore. Other days, like today, she'll just find a good spot to burrow in the grass and just plop down and stay there.

A suggestion: if it's a matter of just making sure your lid is secure, you can always try using gluing in some scrap lumber pieces or, for small enough gaps, some insulation tape (as long as the snake can't reach the adhesive).