These are absolutely gorgeus! I hope you don’t mind me using them. Ofc ill credit you and link to this thread xD
When it comes to Khmer, Thai and Champa.
The Champa is a bit tricky. the Dai Viet pretty much erased their heritage and everything off the map.
The only thing I have:
But I do not think they used that color palette.
Here is colorpalette commonly used in Cham-Textiles, pretty much only cultural remnant left of the Champa Kingdoms, made my the minority Cham people in Vietnam and Cambodia. (Remnants of the Champa kingdoms)
and I also always keep coming across this Champa motif you see on the shield/breastplate.
I think this is also the motif supposed to be on that flag.
When it comes to the Khmer Empire, this was their flag.
(They just had a triangle, kinda like Nepal, or typical signalling flags)
For the thai people. They were as creative as the Abbasid in the beginning. Just having a red flag.
But this account for the Ayutthaya kingdom and onwards into modern thailand.
Ayuthayya often used this emblem aswell
Pre Ayutthaya, the Sukhothai often had this Motif which is used today in they Provincial flag.
and the Lan Na coat of arms was this:
If this is any help.
So for the Champa Kingdom, you’re pretty free to be creative in the design.
The Khmer shud be pretty straight forward.
And the Thai-kingdoms would need to have some creativity mixed in. Figuring out a way to incorporate the motifs. But personally I think maybe just going for the Ayutthaya might as well be good enough. Majority of Thai identity falls into Ayutthaya anyway.