You're lucky that BPs are so big- they cannot fit thru tiny gaps. As a long-time colubrid keeper (& used-to breed some), I've seen some very determined & creative escapes, & it's why I started making my own "tank tops". Had a glass terrarium with sliding screen top- seemed very secure in the frame, but even with the pin holding it shut, there was just enough wiggle room (literally!) in the gap in the track for a tiny hatchling king snake to sneak out, following the gap all the way around. It's like who would ever think a snake would try that- but we have to remember they have all the time in the world to contemplate such things...