Almost anything will work as a mold. Well... don't use a
shoe or you might have a bit of trouble getting the soap out.
Your shoe box and garbage sack will work (I imagine) although it will be difficult to get the waviness out of the edges of the resultant soap. I haven't tried a shoe box or other cardboard box but I'm just wondering if you wouldn't be better off lining it with freezer paper. The box should already hold the soap without leaking okay, so the real problem is how to get the soap out, and how to get it out without stuff sticking to it. If you use freezer paper you can just peel off the FP after you get it out of the shoe box. If you have any trouble with getting it out of the shoe box just tear off the box since you'll probably toss it anyway. Then you just peel off the FP and cut the soap and you've got bars!
Another good "found" mold is a milk or OJ carton. Or you can get a 3" ABS pipe at the hardware store. Ask if you want info on the 3" and can't find the post elsewhere on this forum.
There is NO need to apologize for asking questions!
Everybody is here on the forum because they love advice, both asking for it and giving it. In fact we're all madly enthusiastic about our hobby and if we're not here typing about it we're probably thinking about it anyway.
Please do yourself a favor and post any and every question about any matters you aren't completely certain of. I think I can safely say that everybody on the forum wants your soap to come out the best it can be. And please post your recipe particularly if you have any doubt. We can run it through SoapCalc and check and make sure you have the numbers right and maybe provide other suggestions on the recipe.