Hi Teresa,
I've made them and they would be great for your mini hosta. You can use just about anything that has a firm side, like a milk jug, a woven basket, tupperware like containers. The main thing to remember is you'll need two cotainers for each project. You'll line the larger one with the thickness you want your walls, poke holes in the bottom for drainage, and then put your smaller container in to maintain the size of the bowl area of your project and the walls. I did one in a basket and couldn't get the basket off so I just left it and it like it. Line your "tufas" with a garbage bag or something to they won't stick to the mold.
I can't find the pics of mine but these are some that Tom Bruce of Carolina Daylilies & Hosta made. He puts mini hosta in them and they look great.