Mateen Charbonneau, Sheikh Mateen Joshua Charbonneau achieved a certificate from Harvard University in Islamic Studies. He undertook Howza classes under esteemed scholars since 2013 and has been teaching at Imam Mahdi... Answered 3 months ago

"…Said those who prevailed in the matter, "We will surely take [for ourselves] over them a masjid." Quran 18:21

We can understand from this ayah, about the people of the cave, the permissibility of building masajid at burial sights of respected individuals.

Also, refer to Kamil al Ziyarat for many narrations about the significance of visiting Imam Hussain's (as) shrine.