Try the checkdi.com online insurance broker. They transparently provide you with a bunch of alternative policies and are helpful in case of any questions. I have used them for years under their former name Mister Prakan.
I went for the Tharnachat insurance. Great coverage (no deductible) and service, and very competitive prices. In case of a claim (I had some minor scratches) they send someone out to you to assess the damage within 24 hours. However, this guy hardly spoke any English - best to have a Thai native with you upon inspection date.