I've installed the Wolo Bad Boy on a few bicycles, using a small 12v battery and a momentary switch. With some creativity, the horn fit under the front basket. I put the battery inside a soft case I picked up at the thrift store (originally meant for cassette tapes - remember those?) and wired the momentary switch the right handlebar, near the handbrake.
Is it loud? Yes, yes it is. Plenty loud.
One of these might do it for ya: https://www.amazon.com/Wolo-419-Bad-Boy-Horn/dp/B000F5DQWY https://wolo-mfg.com/horns/motorcycle-horns/air-horn/model-419-bad-boy.html
https://www.amazon.com/gp/product/B000F5DQWY/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
Hope that link works...if not, it's the Wolo (419) Bad Boy Air Horn - 12 Volt
https://www.amazon.com/gp/product/B000F5DQWY/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
Hope that link works...if not, it's the Wolo (419) Bad Boy Air Horn - 12 Volt