There a numerous articles on how to make plaster molds and most deal with how to make a mold for a small ornament or a copy of the body part of a child, but what do you do if you want a large object copying. The normal cardboard boxes used will not be big enough, and it is likely that the normal every day utensils you need will be big enough.
There is no point using a small container thinking as soon as one lot of plaster has gone in you will start making more. Although drying is usually an overnight process depending on how long it takes to make each batch you may find that you have to wait much longer than you thought for drying. Where you will make the item can also be a problem. Can you find a space to work where you will not be in anyone else's way and where you will not cause damage to any of your other possessions?
Having to carry utensils full of plaster of Paris across a highly polished floor or a new carpet is fraught with problems. You may end up causing a damage costing more than the money you think you are saving making the item yourself. Just think how easy it would be to knock over an ornament or worse still something that could not be replaced. There is also the cost to making big plaster molds. Will you find that the amount of time and effort and money you put into making the mold was not worth it?
It may have been cheaper and a lot less stressful to just go and buy another item from the store. You could ruin the whole endeavour by finding out half way through that although you though t you had bought enough materials you run out part way through. If the item is for yourself, time will not be a problem but if it is for someone else for maybe a birthday or Christmas present, will you have time to decorate? It is not just the time making the object but all the extra work that needs to be done afterwards.
There is also the issue of whether or not the item will be suitable. Friends may admire an item you have in your house, and however much they might want one of their own, where they live may not be suitable to house it. It would be a pity to make the item the same size as your own and then realise that it is not going to be of any use, when making a smaller version would have been much better.
There is no reason why you should not be able to make bigger molds at home, as you become more experienced and used to judging the quantities of materials that you are going to need, but it will be best to make sure you make your first plaster molds small and easy to handle. Everything requires time allowance for learning and practice.