Ever wonder what happens to all those used Scotch-whisky barrels once the distilleries are through with them? Up until now, huge planters and awkward-looking furniture were among the most viable solutions. But Daniel Marshall, who has been making luxury cigar humidors since 1982, had a better idea: Why not turn them into humidors? After all, single-malt whisky and cigars are one of the world’s classic pairings. But specifically, it was The Balvenie 21 Year Old Portwood casks that caught Marshall’s attention. A…