Place two pairs of round bricks 2 studs apart
Place a 1x4 plate on top
Put two pairs of round bricks on each edge of that
Repeat
Event the plates:
OnPlayerTouch | Player | Add/SetVelocity | etc.
Text-based model:
_ _
| | | | <-- Round bricks
|-,----,-| <-- Plate (to be evented)
|-| |-| <-- Round bricks
'-' '-' <-- Round bricks