It depends what scale you're building. If using a real Lego brick, I'm not sure there are figurines tiny enough to match the proportions of the characters when they climbed the Lego brick in the movie.

If using figures larger than that, you will have to build a Lego brick from scratch; up until a certain point. Lego makes large storage containers shaped like Lego bricks. You could probably use 1/18 or 1/12 scale figures to match the scale of those bricks. Unfortunately, the underside of those storage containers do not have deep grooves like a real Lego brick; on the top of the container has the studs.

I think you will have to build a Lego brick from scratch.