No external keyboard required. In fact, everything you need is already there.

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Method 1 (recommended):

1. Open up the controls menu
2. Go to other controls
3. Find dev console and hit the button to bind it to a mouse button
4. Do a 3 finger tap
5. Whenever you want to open it, touch the screen with 3 fingers!

Method 2:

1. Open up the controls menu
2. Above other controls, tap the ... button
3. Hit open dev console