YOu can make a leather bag in a bottle neck shape, with a wooden stopper to hold your shot, if you want to go cheap. They are authentic, and work particularly well in the field. Keep the bag small so you don't walk with a limp. Make your own powder and shot measure for a fixed amount, when you decide on a load. Many shooters still use the same volume of shot as they use to throw a given measure of powder, so one homemade measure, made from antler, or wood, or horn, does the job. Just use a scale to check your weights so you know what you are actually shooting.
I have the Irish snake, two of them, as I bouht one from TOTW years ago, and put it on an elkskin shot bag I made. The bag is too big, and carries way too much shot, but no one says I have to fill it, do they? I may cut it down some day and resew it, but for now I just keep about a half pound of shot for the field. I found that the scoop in the measure is not a reliable indicator of the amount of shot thrown. I am currently using an original powder, shot adjustable scoop in which I pour the shot and then add more to bring it up to a truer figure before loading the gun. When I am satisfied with the patterns I am getting, I intend to make a fixed shot measure, and use that. The scoop works to open the gate on the Irish measure, so I can pour the shot out of the snake, but my bag with an open mouth and a stopper will be easier, and probably a little faster to use in the field. I don't know if I will be able to throw the same volume of powder and shot, yet. So, I may still use my adjustable measure for measuring the powder charge, and the separate, fixed measure for the shot. Tuning in a smoothbore takes a lot of time at the pattern boards.