Unconstitutional? Haven't we got past silly concerns such as that? How much compensation was paid to owners of formerly legal supplies of opium, cocaine, marijuana or other drugs when they were declared illegal? That's right, zero.
Once upon a time, people recognized that the Constitutional imposed some limits on their ability to use the power of government in an attempt to remold the world to their liking. Drugs, machine guns, short-barrel shotguns were all legal, and there's no authority to change that, so just impose a tax. After a few years, everybody accepts that the real porpose of the tax was to effectively outlaw them, so constitutional authority be damned, just outlaw them.
You want compensation? I got your compensation right here.
Ken