Get a toolbox that fits in the bed. When I drove my ranger as my primary vehicle I kept my hvac tools in a toolbox designed for the bed, worked out just fine. Make sure your toolbox is water tight! You can also find contractor caps for the bed that have tool storage built in, probably not going to be cheap unless you find someone who really wants to get rid of one. As for under seat storage, not sure if there is even enough room for it, in my ranger at least there isn't much room.
