Hi duncan,
to find out which capacitor and resistor you'll have to install can be found on several web app based crossover calculators.
This is not a real problem so. You'll have to know the impedance and the SPL (sound pressure level / db) from the horn driver
you wish and also from the full range speaker. The level of the full range Jensen can be omitted as the resistor network can be
used to match the SPLs.
I would recommend to use a PA Horn driver. Piezo Horns often sound very harsh and they can scream like a pig..
Real horns do the trick. They are sometimes a bit pricey, but i'm sure there are also good valued horns on the market.
Here is an example for a driver which could be useful:
http://monacor.co.uk/products/speakerbuilding-padriver/vnr/105300/one typical 90-60 Horn:
http://monacor.co.uk/products/speakerbuilding-padriver/vnr/110700/This are only examples. I think you'll have to find some horns in your region.
I have only sources for suppliers in germany as i had to buy two horns for my Yamaha PAs last week. Sry!
Are you from UK? The Monacors are available at the maplin webshop (UK)...
I don't know about the quality now, but i remember that i used some Monacor tweeters 20 years ago for my
DIY teen party speakers. They worked OK but i don't remember how they really sounded.
For the Yamahas i've bought a pair of Celeston. Maybe that's also Monacor...
Have fun

.