I use one of these and love it:
Gunslinger's Corral Of course, if you are not using a bolt rifle, this probably will not work. I've been able to alter it a little and use it with a pack, as well as without and find it to take away most of the fatigue of carrying a gun.
As far as plugging the barrel to keep stuff out, I stick one of those foam ear plugs in. I've always remembered to remove if prior to shooting the gun. Not sure if it would cause an over pressure issue but assume the pressure would just blow it clear. I've never used tape because I don't want the adhesive residue on my barrel.