I'm looking for yeshiva-style shiurim on Maseches Megillah, esp. the last perek, and more esp. the sugya of kedushas beis haknesses. Can anyone recommend either an online link or a specific sefer?
Not sure if this is what you are looking for but Daf HaShavuah has many shiurim on all the dapim of Megillah. One amazing shiur is from Rav Moshe Aharon Friedman, a maggid shiur in the Mir. He gives 3-4 shiurim per daf, each one covering about half an amud, each about ten minutes long. He reads the gemarra and shares the essential raid of the Rishonim and Achronim.
I can't give a direct link to them all but the first is here.
The rest can be found by signing up at https://dafaweek.com and going to Resources > Shiurim. Or you can take the first link I shared and guess the links for the other shiurim.
They have other Maggidei shiur but I don't know their style.
There's also Shas Illuminated, which says they have iyun shiurim. Not familiar though.