My basic idea is a horn that produces food which if consumed, gives various illnesses. The severity of the illness is directly proportional to the size of the food issued. For example, a small ham sandwich or apple would cause a cold. A full turkey would cause a case of Ebola or some such thing. I have no experience of writing SCP's but I would like to start. Any comments?
They sound more like ordinary sandwiches, laced with harmful viruses/bacteria than anomalous artifacts.
I think the best way to write an SCP is to think of something that frightens you in particular. Than find a way to convey that fear to the reader, so that they may feel it. This means marrying an object/location/thing/person with that fear.
It is also important to note that an SCP is essentially telling a horror story: this means you need to begin with a set up (description of the object) to set the scene and atmosphere. Then you need a trigger - something which sets the horror off (usually something mundane enough for an everyday person, like the reader, to do). Then you have the pay-off; the horror. Horror works best when left to the imagination of the reader, so you should provide as scant information as possible to facilitate that. Use [REDACTED] to that same effect.