Matt,
In spite of my rant, IMO, there is certainly nothing wrong with playing around with some 15's to see what you can do with them. I'm probably the first in line when it comes to doing that.
I'm simply saying that using the 15's in a DIY horn isn't a good way to "open up my options". There are too many guys that plan around using mismatched drivers, cabinets, and other sundry gear to make gigs happen. Sometimes when I have rolled the dice over the last 30 years, I've had good luck, but there are other times where I've wasted time, effort, and costs that could've been better applied to other equipment. Instead of wasting time on a DIY, I could've been earning the funds doing tech work, repairs, or cabinetry for a client.
I'm sure you know this already, but.. your best bet: figure out what you need to do. Get a list of models and makes together that meet the criteria. Price them out. Overcome the sticker shock. Find a way to finance, earn the funds, credit, whatever. Buy once, cry once. Enjoy the gear.. instead of worry about it..
Regards,