Take a case over to your dads, with a fired primer in place of course. Pour in a charge and mark the case. Cut off the case and debur the case mouth. Check the capacity of the case by filling it with powder, using a funnel and the can. Do this in a round 8inch cake pan so you can collect the stray granules. You might have to trim the case of course, with a file.
When you are satisfied that you can pour the charge within a tenth of a grain, one way or the other, you are set to go. Do this for each charge you want to create.
This is a tried and true method, used by many for years. If your load is at maximum or close to it, you may not want to do this. Usually the 45-70 isn't happy with maximum loads anyway. A tenth of a grain one way or the other in such a large case will make little if any difference one way or the other.
If you're hell bent on using plastic containers for charges, go to your local pharmacy and ask to buy some of their small plastic pill bottles with easily removed caps. If you know a diabetic, ask him/her for their used test strip bottles.