Couple of properly sized hose clamps will work. Or Tv antenna clamp, depending on the size of the pole.
Antenna clamps
That link offered some great alternate clamping solutions I haven't considered before - thanks! To the OP one thing I can say is sit down
pen to paper and really think what the long term plans are. Makes no sense buying a small enclosure and later find out you ran out of space or its too shallow etc.
Obviously, one has to balance costs vs wants vs needs . . .
Regardless, once you have the proper size almost anything can be added / modified to suite your needs. Many of the more expansive boxes incorporate all manner of I/O's such as 120 VAC outlets, fans, exit ports. All of these can be added with some sweat equity if you're handy.
Having said this some key things in mind enclosures which incorporate those grid points are not just a nice to have but a must have. As they allow you to mount anything with a simple screw to affix the same. An enclosure that just has flat backing and just a four point mount restricts your ability to
Plug & Play.
Even in those cases its not the end of the world as anyone can just add in a piece of wood / plastic / DIN rails etc.
As noted up above any enclosure can add in extra waterproof glands / bushings from aliexpress / amazon. One thing to be cautious about is (IF) you believe you need to vent the enclosure. It's highly recommended to purchase one that is vented properly vs just drilling a massive hole and sticking in a fan.
Almost every DIY enclosure I've seen where people have done the same ignored basic safety and it made the whole thing a ticking time bomb. Never mind all it did was shorten the lifespan of the hardware inside.
Some key take away is anything that's intended to be installed outside be properly outdoor rated. That covers temperature, humidity, and UV rating. You buy just any random box and stick it outside it will be fine for the first year but after the next the extreme cold and heat and especially UV will eat a none UV rated box into toilet paper.
If the enclosure can be placed in the shade it will extend the service life of the enclosure and the components. Again if none of the equipment is outdoor rated (hardened) than special care must be taken to insure a long service life. Moisture is one of the biggest killers besides heat as it relates to electronics.
Go on aliexpress / amazon and buy a few large silica desiccant packs and stick it in there once the install has been finalized this will keep the RH more stable. When a sealed enclosure is used and you're still worried about heat adding a few cheap aluminum / copper heat sinks is cheap and effective to dissipate heat.
If required in a sealed enclosure adding in a small PWM fan that is thermostatically controlled also helps extend the life of the hardware.
Love to see how it all ends up once you're done . . .
Good Luck . . .