Those thin ones work for some people at my gym. If you find that they're too thin for you, you can also try the shin guards with the velcro that loops around (instead of 2 separate velcro straps), as you may be able to tighten them more. This is the type of strap I'm talking about.

Another thing to keep in mind is that your calf will get worked and grow from throwing kicks, knees, etc. in Muay Thai, so you might grow into them eventually.