There is a way to make planetary mass black holes. This method does not rely on high pressures to compress mass into small volumes. All you need is oodles (and I mean really a lot) of kinetic energy and good aim.
Say you want to make a black hole with the same mass as the earth. A black hole of this mass is about the size of a marble. What you need to do is take two marbles and accelerate each to a velocity such that their kinetic energy has a mass equivalent of approximately the mass of the earth, but have their velocities opposite, so that the net momentum is zero. Then have them collide head on. Being marbles, they can get pretty close together before colliding, well within their combined event horizon. Bricks with the same kinetic energy probably wouldn't work since they would collide and splatter too soon. BBs with the same kinetic energy would be even better, but would probably be harder to accelerate to the same kinetic energy for the same reasons that electrons are harder to accelerate to the same energy as protons.
Imagine two BBs coming together in a near miss scenario, passing each other within the diameter of a marble. They would become permanently bound orbiting each other at high frequency, emmitting high frequency, centimeter wavelength microwave gravitational radiation. Imagine radiating planetary mass equivalents of gravitational energy within the time span of a few seconds (shudder)!!
As an exercise, let's calculate what a future advanced civilization would have to do to make an earth mass black hole like above. Assume they build 2 100 light year long accelerators end to end, with the collision chamber between the two.
1. Mass of earth 5.98 * 10^24 Kg
2. Each accelerator 100 LY long or 9 * 10^17 meters.
The accelerator would have to dump 6313 metric tons of energy per meter of accelerator into the marble. That's a heck of a lot of energy.
If we made the accelerators a 1000 times
longer, then the project would take at a theoretical minimum 200,000 years,
but then we'd only have to dump 6.313 metric tons of energy per meter of
accelerator into the marbles. Much easier! A more time and energy efficient
method would be to build many accelerators radiating outward from the collision
chamber in 3 dimensions like pins in a pincushion. Then you could collide
multiple marbles (or BBs since they are smaller) at the same time.