Surface normal perpendicular direction

local M = game.Players.LocalPlayer:GetMouse();

function Part(Orientation)
    local P = Instance.new("Part", workspace);
    P.Anchored = true;
    P.CFrame = Orientation;
    P = nil;
end

M.Button1Down:connect(function()
    if (M.Target) then
        local Surface = M.TargetSurface;
        local HitPosition = M.Hit.p
        local WorldSpaceNormalIdVector = Vector3.FromNormalId(Surface);
        local ObjectSpaceSurfaceDirection = M.Target.CFrame:vectorToWorldSpace(WorldSpaceNormalIdVector);
        -- ObjectSpaceSurfaceDirection is the direction the target surface is facing

        local Up = Vector3.new(0,1,0);
        if (math.abs(ObjectSpaceSurfaceDirection.Y) >= .99) then
            Up = M.Target.CFrame:vectorToWorldSpace(Vector3.new(1,0,0));
            print("Target surface faces world-space 'up.' Switching world-space 'up.'");
        end

        local PerpDirection = ObjectSpaceSurfaceDirection:Cross(Up);

        -- Create manual CFrame for part to "lay" on the surface.
        local FrontAxis = ObjectSpaceSurfaceDirection:Cross(PerpDirection);
        local Orientation = CFrame.new(HitPosition.X, HitPosition.Y, HitPosition.Z,
            PerpDirection.X, ObjectSpaceSurfaceDirection.X, -FrontAxis.X,
            PerpDirection.Y, ObjectSpaceSurfaceDirection.Y, -FrontAxis.Y,
            PerpDirection.Z, ObjectSpaceSurfaceDirection.Z, -FrontAxis.Z
        );

        Part(Orientation);
    end
end)