First off, we should consider what it means to be racist. You've suggested that Lula is "racist" toward Jem and Scout , but that's not exactly a fair assessment of the situation; a more accurate way to explain the incident at the First Purchase church would be that Lula exhibited...
First off, we should consider what it means to be racist. You've suggested that Lula is "racist" toward Jem and Scout, but that's not exactly a fair assessment of the situation; a more accurate way to explain the incident at the First Purchase church would be that Lula exhibited prejudice towards Jem and Scout. So what's the difference between racism and prejudice?
Prejudice is a preconceived notion that is not founded in reality, while racism is a system of disadvantage perpetuated by discriminatory beliefs in which one race profits off the oppression of another. As a black woman in 1930s-era America, Lula is not in a position of power and is, in fact, subject to the tremendous racist beliefs of the surrounding white community. She is not behaving in a racist manner because she has nothing to gain from being complicit in a system of racism; she is the oppressed, not the oppressor. She is, however, behaving in a prejudiced way, conveying her disdain for white individuals by suggesting that Jem and Scout should stick to attending a white church. I'd argue that there's good reason for this kind of reaction, since white people have traditionally posed a great threat to black communities, but we won't delve too far into that. This is a complicated conversation that certainly extends beyond the limit of this context, so to summarize: prejudice is individual, while racism is both individual and institutional.
Now that that's cleared up, let's answer your original question! A good physical symbol that represents Lula would be a bear; she is overprotective of her "territory," suspicious of Jem and Scout as "intruders" in that space, and hyper-aggressive when she senses that they may be a threat or that they are somewhere they don't belong. Her height ("She seemed seven feet high."), stature, and confrontational manner all bring to mind the image of a bear rearing up onto its hind legs, preparing to attack.