Just curious. Why flip it over? Why not leave it hollow? Just start at the bottom of your form and work your way to the top. If you want a solid bottom, make a base of your mix and sink your form into the base.
If you do two batches and flip it over, make sure you keep the first mix damp. Don't wire brush yet. Before applying the second part, spritz with water and add your mix to the other half. You can make a slurry out of the second mix and apply a thin coat over the first mix. Some people use a bonding agent, I don't use one unless it's an old piece. Let it set up for a short time (could be any where from 1/2 hour to a couple hours depends on your mix and the weather)and then wire brush. Rock on! Billie

Billie,
Roaches don't care for water. We things would not be their environment of choice unless there's a food source nearby. They live in cardboard boxes like the ones people use to store things in under the cabinets. They live in grocery bags and in the corregation of grocery boxes. They live in the cracks between the wood in walls, and IN walls. Most importantly, they live near food.

I think your rocks out in the garden with be safe from roaches. Decomposing newspaper over time? No.

But your rocks sound great!
Christine

Cockroaches do like and need water - they love dampness. That shouldn't matter once the rock is outdoors as damp newspapers won't last very long (worms love them, too).

I thought billie anne's idea was good.

Some people use spray foam insulation instead of newspapers - it's a bit firmer to press the tufa against.

Here is a link that might be useful: cockroach page