As others have mentioned, sak yant aren't really "for" Muay Thai. In fact they are seldom prominent in most prominent Thai Muay Thai fighters. They are kind of from the under-class of Thailand, much as perhaps tattoos in general were in western countries for a very long time. And, some of that under class become fighters. But...mostly they are just symbolic representations of protection, or power, believed in an an animistic level. A sak yant of a tiger may just be summoning up "tiger energy", the ability to command, stalking forcefulness, etc. It could apply to anything in life.
So...sak yant could just tap into or express the underfeelings of what Muay Thai has brought to you, without being some sort of "bro" appropriation. Your own attempt to get in touch with that thing, that meaningfulness. Or, maybe not.
In either case, I would say to just get in touch with that thing that Muay Thai has done for you, brought to you, and then find some representation that speaks to that for you. Maybe its nothing that looks like its related to Muay Thai at all...but YOU know it's about Muay Thai. Or, perhaps, if there is a particular heroic fighter who inspires you, then perhaps something related to their image.
As for words, Sylvie's discussed a series of Thai words that embody the spirit of Muay Thai, I'm sure she would double check the graphic for you before you got it tattoo'd. This was one: Ning:
https://web.facebook.com/sylviemuaythai/photos/a.134623809905091/2636651226368991