What PCB coating is good for a high temperature environment (my environment is about 30-70 C), and is available in a spray can or so that is easy to implement without need of a device/machine?
There are silicone based conformal coatings, available as spray. They should bear this temperature range easily. Polyurethane based sprays may be used, too. We had tests running with PU. It proved usable up to 90°C. But all depends on the actual use. You can buy those materials manufactured by several companies like CRC industries, electrolube etc.
If you want to increase creep resistance by using a conformal coating, you will have to do several tests anyway.
Humiseal makes a wide variety of coatings, many of which would be suitable for your application.
30C to 70C is not a particularly demanding environment for most coatings - I have used various types in avionics environments, where card edges are expected to get as warm as 85C (with attendant higher temperatures under toasty components) with no issues at all.
The most common one we use is 1B31S.