Could this be possible, could a soul actually exist inside crystal?

good one to ask that question in medication is Merlin.

spirit energy flows freely in the omniverse. restricting freedom is not a good thing to try and so i'd not bother. better to consider, can you capture the reflection of a man? a photo does this and is useful to a limit.
Science is working on cloning animals, again with limited success.

Copies can be produced of anything and yes a good way to do this with a spirit is in crystal. think the ancients used pure gold.

Voodoo used a jar to capture spirits and this is done during the time of dying. so you may choose to look down that path, though i'm sure better safer and smarter options exist.

some cultures use metal, led if I recall correctly, to store a spirit. It didn't work as a prison situation but rather slowed flow while a deal was conducted in form of a trade. typically the spirit would request energy in exchange for a life time of assistance (a lifetime for a human is short for a spirit and a greedy spirit may think this is a deal) I believe the making of a family sword in some asian cultures used this.

Another option is to build a spirit tool. A tool that is not physically real but floats about in your auric field. Much like a computer program is not a physical thing. If enough years of dedication are placed into making the tool it will serve a person over many life times. typical spirit tools are a diamond (crystalized spirit energy through a refinement much like making alcohol) a crown, a sword, heck almost anything you would see on a tarot deck has been used through history as a spirit tool.
To work out the best spirit tool to make start with self analysis through a spirit mirror. look at your inner spark and determine your best spirit career path (teacher guardian lawtester watcher) then determin which tool will help you best in that long term goal. physically make the item in this world as a kind of template mold and work with it.,

Best of luck and do keep in mind that there are always many solutions to a problem. No real solution involves imprisonment at any level. That kind of thinking with spiral a person to the darkness of elemental matter...

Or you can accept that I am crazy and all of this is the rantings of a story teller.