I would like to know how to secure the PCB into a watertight die cast aluminium enclosure that has no visible mounting posts. There are plenty of those on the Hammond website, and I don't understand how they are meant to be used.
It has to be IP65, and reasonably vibration resistant for mounting on top of a high tower, and resist about -10°C to 75°C inside.
An example is the Hammond 1590W Series.
Different Hammond's enclosures have got different ways of mounting PCB.
The 1590W series has got vertical PCB card guides on all 4 sides.
From datasheet from here.
This sort of housing assumes the pcb will mount perpendicular to the cover. That is, the card is just as wide or long as the inner dimension, and slides in between the ribs around the edge. These ribs, and their spacing, act as edge guides and supports. This means, of course, that your pcbs will be pretty small.
Alternatively, you can use a larger board which fits parallel to the cover. In this case, you drill holes in the back face, then mount standoffs which then connect to mounting holes in the pcb. If you do this, it's important to place elastic washers between the standoff and the back face to form a water-tight gasket for the screw hole. Generally, a metal washer between the standoff and the elastic washer is a good idea to spread the mechanical load on the gasket. Alternatively, if the back wall is thick enough, you can drill and tap for screws which will engage the standoffs.
I normally do not answer questions here that are basically shopping questions that can be solved with Google searches. However in this instance I want to point out that there are die cast project boxes that have very nice PCB mounting provisions if you look for them. Camden Boss is one company that has the type of enclosure that you require in a variety of sizes.
Here is a picture of one of these boxes:
You can see the board mounting turret in the corner. A nice feature of this box is that it has more than just four cover screws so that you are assured of a better lid seal to the gasket.
Here is another picture showing that for this size box the mounting turrets are in all four corners. (Some of the smaller or narrow enclosures may only have two mount points on opposite corners).
Note: I have no affiliation with Camden Boss and have not used their products. Pictures borrowed from one of their freely downloadable PDF box data sheets. Showing the power of Google searches.