I’d like to keep this going for a while and see where it takes us.
For 803, my mnemonic image is a foam fire hydrant. 
(Do you guys call it something different in the States?)
In my (slightly adapted) Ben system,
8-0-3 = f/v-oh-m, so “foam”. I find that easier to remember if it’s coming from a hydrant.
It’s quite a rounded looking number and a nice, round-sounding word, so I make the foam all bubbly and big, perhaps enveloping other aspects of the image.
What about you?
Fire extinguisher.
Fire hydrant pretty much exclusively refers to the unit outside by the road connected to public water supply that fire fighters can attach their hoses to.
803 for me is FSM, the Flying Spaghetti Monster. One of my very few exceptions to accept acronyms. I read it phonetically as “fism” but thats the image it conjures.
Interesting, to me 803 does not look round at all, and checking with $ factor 803 verifies that yes, it’s 11 * 73, which seems very prime to me, the complete opposite of round. I assume you’re using the shapes of the numbers, which is interesting to me
Yep. I didn’t even know some dialects called fire extinguishers fire hydrants until today
Re the OP, I’m still filling out my major system, and I don’t have anything for 803 yet. Some quick brainstorming is not yielding anything great; the only phonetic hit I’ve thought of is Phasmophobia, but I’ve never played that game, don’t watch streams of it, and really only know it from stream thumbnails
Face me
Not to be confusrd with pansy 920
Ah shoot, I meant extinguisher. ‘Hydrant’ was a bit of a brain fart on my part. Knew it didn’t feel quite right! 
We don’t normally have overground hydrants in the UK which I think contributes to my occasional confusion between these two words.
Re: roundness, yeah I meant literally the shapes of the digits, nothing to do with mathematical properties (which I don’t tend to think about that much).
With me 8 0 3 = is something with a freemason, since it makes me think of it
803 = vesiämpäri (water bucket)
face mole
My image is foam too. (803 = FOM)