There are 3 options that have reasonable security. By reasonable I mean it is a locked hard cover, not fabric that can be opened with a knife. Those options are: hard folding cover, tonneau, cab high cover. I had a cab-high on a previous truck and it provided great space, was completely dry, but visibility was poor. On the truck I have now I didn't need as much space and wanted better visibility, so I put on a folding cover. Visibility is great, it's dry if the seals are good, but space is limited. Access is better than with a tonneau but there are times I wish I had the extra storage space a tonneau provides. The folding cover space is adequate but tight when on a western hunt with a buddy.
My folding cover has hinge seals that need to be replaced periodically to stay dry inside. I had to replace them after 4 years when the truck was parked outside every work day. Once I retired and parked in my garage (12 years ago), I have not had to replace them but will need to in another year or two. Unfortunately, the mfg is out of business. I think I will replace it with a tonneau.
You really need to decide how much space you need. With a cab-high, there is enough space for elk antlers, camping gear, etc. Also, visibility is less of an issue if you have the back-up cameras that are now common.