This page is relevant for all the projects that require use of floral foam inside the crocheted box shape.
If you don’t add weight to the bottom of your project, it could become top-heavy and tip easily. The solution: stick something heavy inside at the bottom. A square tile of the right size would be handy for this, although lots of other things would work.
For my first foam block, I used a random metal piece that we had stored in the garage; I gouged it into the bottom of the floral foam.
I'm a frugal crafter, and that was the cheapest solution I could think of. It's not too elegant, so if you'd prefer a prettier solution, you could do the same thing with a few glass marbles.
For my second project, I used a rock as shown. I used a spoon to carve out a bed for the rock.
If all else fails, you can wash off a few rocks, dry them, and embed them in the bottom of the foam like I did. If there is any possibility that an inquisitive person might pull the whole thing apart to see what’s in it, you can glue a square of stationery, scrapbooking paper or other pretty paper onto that end to hide the mess. If you don't, you will be able to see the rock (or other item) through the bottom of the crocheted granny square, and it may even fall out. That would be bad.