As I know now there is not standard Makruk notation in enlish
This web has Makruk to play online that can save pgn
http://www.playok.com/th/makruk/
it use chess notation with some adpation, all pieces use like chess, but only promotion pawn to Q they use e6=P and after that they use P and for this pieces which has same function as original Makruk queen.
this web is Thai web to play online that can save pgn also, but all notation is on Thai charactor.
http://www.thaibg.com/TSOnline/index.php
Thai notation is like chess notation in full form, by example if we move N from b1 to d2 needs to write N, b1-d2 but all write in Thai charactor
I would like to suggestion Makruk notation in English should write as chess in short form like http://www.playok.com/th/makruk/ doing, but should change something following
1. B to S as it moves like Silver in Shogi
2. Q to M as it was called Med in Thai and M is not in a-h file notation so it will easy to read
3. pawn after promotion should using as same as original queen notaion which may be like this 'e6=M'