How would I specify a specific surface of a union?

I need to specify the bottom surface of a union for a verification method, however at the moment I am using a separate part - kinda causing the physics to go a bit unpredictable.

Instead of the union on its own getting touched, how would I specify the bottom surface?

game.Workspace.WBM.Union.Touched:connect(function(hit)
    local find = hit.Parent:FindFirstChild("top")
    if find ~= nil then
        part.CFrame = CFrame.new(12.37, 2.98, -19.72)
        win = false
        part.Anchored = true
        game.Workspace.WBM.Part.Anchored = true
        go = false
        start = false
        wait(0.01)
    else
        win = true
    end
end) --- need to use variable, left from movement script somehow

wait(0.01)
end
wait()
end