Hmmm. I had a better look and they list the output of the Modcharger as 1 amp @ 12v. So not sure how well it would power a horn then... the matress inflator would have to pretty small too.
What about an air horn? This one says it's 115db. You pump it up with a bicycle pump.
https://www.amazon.com/Delta-Airzound-Bike-Horn/dp/B000ACAMJC/ref=pd_bbs_sr_1/105-2339420-5913210?ie=UTF8&s=sporting-goods&qid=1194517539&sr=8-1&tag=viglink20253-20