I want to say: "I'm not used (accustomed to) to hot weather".

My Thai dictionary defines "accustomed to" as คุ้นเคยกับ (khun khuui gap).

So would I say: "Pom mai khun khuui gap Agad rawn"?

You can say that. I use ชินกับ as it's a bit shorter:

ผมไม่ชินกับอากาศร้อน

pŏm mâi chin gàp aa-gàat rón

Either mai khun khuui or mai chin can be used

Sent from my SM-G925I

ผมไม่คุ้นเคยอากาศร้อนครับ

Pom Mai Koon Kuui Agaht Rawn Krap

I'm not used to hot weather....

Apologies for the iffy romanisation but 'เคย' is one of those tricky 'unromanisable' words so I went along with the general usage here.

I think keui, is more like the 'experienced particle' eg keui bpai pattaya, mai keui

mai keui yuu tii pratet raan gern-bpai

i'm sure i could be wrong

ผม pŏm I (used by men only) ไม่ mâi no ; not คุ้นเคย kún koie be familiar ; intimate ; acquaint อากาศร้อน aa-gàat rón hot ครับ kráp (particle used by male speakers at the end of sentences to make them more polite)