I also have made many, many powder horns over the years. And I am also confused by what exactly you are asking. Normally, the butt plug is sealed in and secured by some means - tacks, nails, I've even used locust thorns. I am now talking about the plug on the open end - that portion of the horn that is from the "head end" of the cow, buffalo or whatever. I usually turn my plugs, boil the horn to soften the horn, and push the plug in. I seal it with beeswax. SOME horns have plugs that have a removable screw in finial that usually attaches to the strap which can be removed for filling. MOST horns do not. Normally, the horn is filled from the "spout" end. This can be done by the use of a small brass funnel - I just roll a piece of paper into a cone, tape it, snip off the point and use that for a funnel. For safety sake, you never want to pour directly from the horn inot the muzzle - you pour from the horn into a measure and then pour from the measure into the muzzle. The purpose of the horn (or powder flask) is to "keep your powder dry". That being said, you want your powder horn to be as air tight as possible so that it stays dry and doesn't pick up humidity from the air. The "spout" end of the horn is sealed with a stopper of some sort. I utilize used "fiddle pegs". (I also restore fiddles). I use a tapered reamer to taper the hole in the spout end which matches the taper on the fiddle peg. Not having those things, a stopper can be made by whittling one out of a hardwood dowel, etc. Powder horns can be as simple or as decorative as you want. I've scraped them so that the horn is almost transparent so you can see the level of powder in them and I've made some and left them rough. In essence, they are a "utility item" with the purpose of carrying the gunpowder and keeping it dry.
In regards to the drum tightness on a CVA - can't answer specifically as I've only had one CVA. Normally the drum has a "shoulder". This is intended to be bottomed out on the flat of the barrel - the end witht he threads on it that screws into the barrel should be the length that comes to the inside bore of the barrel - if it extends into the bore, it will catch wiping patches. If it is too short and ends before the bore, it will accumulate "fouling" which can be dangerous. If fouling is collected there, it can hold an ember after the rifle is shot and even wiping the bore between shots with a spit patch, the ember can still remain - this can cause the next powder charge to go off when introudced into the bore. A drum should never be "loose". If you are having problems with your drum and are unsure about how it should be, I'd suggest you take it to a gunsmith who has experience with muzzleloaders and have him check it out and show you how it should be. If my memory serves me right, on some CVAs, the drum is "locked in" with the breechplug or vice versa. They have to be removed in the correct order or you can cause some damage. Hopefully some CVA owners will chime in here and clear that up and be able to help you out.