Textual: It would appear that there is a symbolic meaning attached to the two days and two nights of the appearance of death. As noted in the comments on Alma the Younger’s similar experience (see the comments on Mosiah 27:23), the two days and two nights yield four time periods, and four is a ritually significant number among most Mesoamerican cultures. It may be that regardless of the actual time spent in the coma-like state, it became two days and two nights in the writing because this lent added significance to the occasion.
Cultural: The placing of the body in a sepulcher is an authentic connection for the known Mesoamerican populations, particularly the Maya (Simon Martin and Nikolai Grube. Chronicle of the Maya Kings and Queens. London: Thames and Hudson, 2000, p. 19). However, it is known only for the kings or highest royalty. Many Maya kings were laid to rest in the hearts of stepped temples. The most famous of these is the tomb of the king Pacal who reigned in Palenque. The cover of his sarcophagus is a beautiful example of Maya art, but has been variously misinterpreted, included a most fanciful description of an astronaut in a space capsule.
Pacal was the first of the entombed kings found, but others have followed. Thus when we read that they are ready to place King Lamoni in a sepulcher, we may imagine what that would be in a Mesoamerican context. During his lifetime he would have built a temple, and had built into the temple the chamber where he would eventually lie. As a king, he would have the right to be buried in such a tomb, and would have had the wherewithal to have it ready, even prior to his death. These details fit with the limited information that we have in this verse.