I am a lefty and used Club Champion here in Birmingham, AL and what I liked is they use a 6 iron. In my humble opinion, if I can hit your hardest to hit irons which long irons the rest of the bag can usually come together. I would also stay away from 7 irons because the club manufactures know this is the testing club and are really jacking around that club’s basic design.
A different approach to demoing is buy single irons off eBay from a reputable seller. Make sure the seller tells you all specs before buying. Also if you know some of the basics of club fitting or some of your own specs can help with this process. I believe Club Champion club fitting can be $300+ for a full bag fitting and that kind of money can buy several single irons to test as long as you like. Also most golf shops will be happy to confirm the specs of the test clubs for free and if a lie or loft adjustment is needed they’ve adjusted it for free for me.