I’ve found Toolstation provide a range of weather-proof boxes, but they don’t file them very well in their online catalogue, so it is not that easy to find the information.
If you search on “waterproof”, you get a nice range of waterproof coats.
A search of “weatherproof” throws up this “Weatherproof Box IP54 12 x 19 x 30cm” which I used for the 3cm transverter & amplifiers. I added a strap around the middle to help hold it tightly shut. It gets shaken well in the gales. I also drilled some drain holes in the bottom to release any accumulated condensation. The box has a set of rubber glands on the bottom for feeding cables through. The hinged lid means you are not having to fiddle about with screws in order to get at the contents. I added a bracket under the box so as not to distort the dish (not in the picture).
A search for “moulded enclosure” offers these IP55 boxes. I have one on the main aerial pole housing the 13cm transverter (see banner picture above). There is a useful range of sizes and there are pre-cut holes handy for feeding cables through with grommets for encouraging the rain to stay out. Again, I’ve drilled drain holes in the bottom. The lid screws tend to rust, so I’ve greased mine, but stainless steel screws might be better.
I added one of the smallest IP55s to the lid of the 3cm box in order to cover a hole I made.
I have heard of people pumping dry air into their outdoor boxes to keep the electronics dry, but I haven’t tried this.