I now have a tank that is large. Before I got one I used a smaller dish and I Parkerized a barreled AK action using the immerse-Park-move-reimmerse technique. I got several lines in the finish and will have to do it again.
I plugged the muzzle, breech and the gas cylinder using cork. It was probably unnecessary since the chrome will not take the finish. I plan to replug and use the new tank.
You may wish to get a PVC pipe cut to size and capped with the right size cap. I did not check but a 3" or 4" pipe might be large enough to accommodate the height of the receiver. You may wish to place the pipe vertically with the sealed cap resting on the floor and pour the hot Parkeizing solution over the treated parts.
By measuring with water you can make enough to fill the pipe and avoid any liquid surface that might leave a ring on the metal. Keeping the solution hot enough may not be a problem if you do one action at a time.