Is there an appropriate visa to stay long term in Thailand for someone retired but under 50?

Not working, just living on a modest life from overseas saving and investments.(in my case about 40-50,000 THB a month)

P.S. Thanks for the helpful comments on my related post two days ago. There were some interesting suggestions there, but no ideal solution. I also inquired about METV, but the London embassy told me that requires a letter from an employer, so not possible for me.

What I am asking now highlights maybe highlights a fundamental issue: for some people there just doesn't seem to be an appropriate visa for their situation?