the bad news:
The notion that one can wear any kind of very covering costume along these lines outside in even the best weather is insanity, 2 hours tops, with 30 minute breathers with the entire upper torso removed is more realistic. In summer, cut that down to 45 minutes.
The good news:
That doesn't mean you can't make a golem costume for your LARP, just that you must go about it very, very differently than one to be worn indoors just standing around at a con.
First, have lots and lots of tiny holes for air flow, maybe use athletic mesh (1) as a base material instead of some solid fabric like spandex or twill.
Second, I'd do all soft fabric construction (2) and paint in all the mechanical bits. Mostly because this will allow maximum movement without worrying if you are going to break a hose or a gear or whatever. If it's all pillows, no biggie, you're even padded if you fall.
Third, a framework to create bulk is a good idea, and football or hockey pads are a good choice. If you add pieces to the shoulders to broaden the whole body, you can create the golem torso to hang out away from your body instead of hugging it, thus also increasing your wear comfort. (3)
2) http://www.partycity.com/product/toddler+boys+hulk+muscle+costume+classic.do?kwid=t oddler%20boys%20hulk%20muscle%20costume%20classic% 20-%20Size%20-%203T-4T&qcid=&ref=ci&extcmp=pla|Google
not quite that silly, but the concept holds.