Not sure if your looking for a hard cover or a soft cover, but I’ve been happy with the Truxedo LowPro cover I put on mine. I’ll try to link it below, it’s around $500 on Amazon.
I’m not particularly concerned with the security of it, the ole “out of sight, out of mind” trick comes into play here as people can’t just look into the bed of my truck. For me it was about not losing bed space. It can be completely rolled up and you don’t lose an inch of bed space which appears to be the case with some of the hard covers or tri-fold covers. I’ve kept ball gear and such in there for months with no problems. I just wanted to be able to keep things dry when I need too. It can be rolled up in about 30 seconds or put back on about the same time.