A quick way to test for the right amount of weight is to remove the mag tube spring retainer and put some shells between the retainer and the spring. Then weigh the shells on a postal scale.
Since this method also works permanently, especially if you want the weight to be hidden, if you handload you can fill unprimered shells completely with shot for extra weight, and as above insert them between the spring and retainer.
My cheapo postal scale says it is a hair over 11 oz. I am not making them, and this is the only one I have. I suspect I got this off of ebay many years ago. My 20 ga now is a Standard 20, so it takes the regular 12 ga Remington weight. This weight is just sitting in the safe. ...Bob_K...
Just FYI buy a grade 8 bolt large then put it on metal lathe then have it turned to length and diameter of plug used for duck hunting like the plastic one you get when you bought gun. I also had one made to a longer length made my 1100 a 2 shot gun one in chamber one in magazine. Put the weight between your hands keep balance very good. Cost under $2