Yes it is Constitutional!

The question of whether the Conviction of Ex-President Trump would be Constitutional has already been answered, by the authoritative body--the Senate itself.  The Constitution gives the Senate "sole" authority to try officers impeached by the House.  Whatever the Senate says in these regards, is final.  In an 20th century case involving an impeached administration official, the Supreme Court ruled questions related to Impeachment are not "justiceable," that is, not to be resolved by courts, but only by the Senate itself, as decreed by the Constitution.

AND, the Senate has already "ruled," in an actual bi-partisan vote (a few Republicans) that the trial of DJT by the Senate is Constitutional, and therefore Constitutionally mandated.

Leading Newspapers like the NYTimes and Washington Post Editorialize It is Constitutional (and Necessary).

Many sources say things like "most" or "nearly all" legal scholars agree.  The well known Professor Lawrence Tribe wrote an Op-Ed in the Washington Post (behind their paywall).  Meanwhile, the "legal scholars" who believe it's not Constitutional are retired judges you probably haven't heard of.

It takes a tortured reading of the first Impeachment passage in the Constitution to see how Conviction now might not be Constitutional.  But that passage is merely exclaiming that impeached officers are removed, not that all impeached officers much be removeable.  The second clause discussing impeachment makes it clear there is a second penalty, being barred from future office, that could apply to ex-officers.  That was common in British practice, to convict ex-officers so they could not serve again.  US hasn't impeached enough people for there to be a clear precedent, but the precedents that exist are consistent with this authority.

https://www.lawfareblog.com/can-former-president-be-impeached-and-convicted