There are probably 20 of these at this point. I use amphetamine [1] (free on the app store). It has configurable timeouts with optional warnings/notifications, to reduce the ability for your MacBook to melt in your backpack. Can also have it disable when an app closes/download completes/battery percentage, and all sorts of triggers (like bluetooth presence, useful for backpack proximity).
[1] https://apps.apple.com/us/app/amphetamine/id937984704