I have a Pelican "Vault" brand case that is about 12x16x6. Cabelas sells them as pistol cases. I think it was about $50. It is big enough to replace the glovebox I had on my last bike and then some. I carry a thermos, gloves, a bottle of water, sometimes an airhawk cushion, maps, etc. I really missed the glovebox. Sometimes I wish it was bigger but usually not.
It is easy to get my leg over but not big enough for a helmet obviously. My wife likes it as a backrest of sorts. I seldom lock it but it has holes for a padlock. I'm thinking I'd like to attach some spools and use a cargo net or tail pack on top of it. It is super easy to just flip the latches and throw stuff in it.
The Harbor Freight ones are quite similar, but I'd consider one thing- it is black and sits in the sun. It has to have some pretty good material in it to keep from softening or warping or decomposing in the sun. My Pelican box has only been on there for a few months but shows no issues from the sun. I'd say it is "almost waterproof" but I put papers in plastic bags inside it. DD