Donald Trump's longtime friend Bernard Kerik has some thoughts on how he should serve his time if his guilty verdict lands him behind bars.

Kerik -- who served as Commissioner for the New York City Department of Correction as well as the NYPD commissioner -- tells TMZ segregating Trump in a prison facility isn't enough ... he needs his own facility for safety reasons.

BK -- who received a Presidential pardon from Trump in 2020 for numerous federal convictions -- explains Trump wouldn't be a high-risk inmate in terms of fighting guards, inmates, or attempting an escape -- instead, the environment around him would be high-risk.

He says Trump's enemies -- foreign and domestic -- could scheme to pay inmates to harm him ... making him the ultimate target.

He suggests anyone interacting with Trump, whether law enforcement or civilians, should undergo in-depth Secret Service background checks. Additionally, anyone preparing, handling, or serving his food should also be thoroughly vetted.

Essentially, he says the cost of housing Trump with all the necessary safety measures would be 100 times that of a typical inmate.

It's clear where Bernard's loyalties rest -- especially as he tells us any judge who sends Trump to jail is making a purely political move, especially given that some violent criminals are being released without bail.