Rolling pin and placcy bag sounds fine, although the CT may go straight through the plastic if it's a thin bag.
Personally, I rarely use CTs, but they are handy if you are using, let's say less than 10 a month.
Any more than that and you will probably find sodium metabisulphite powder more convenient and considerably cheaper in the long run. Dissolve one level 5ml teaspoonful of SodMet in 100ml of warm water and then use 10ml of that solution in place of 1 CT.
Find yourself a little bottle and the stock solution can be kept for quite a while.