Aye, that ship has sailed long ago, but if you still want to do it, there are quite a few pre-built for cPanel, and one that I know of for Direct Admin. This place has a solution for both:
http://www.ihostdev.com/ipanel.html I don't know how good it is, I haven't ran a freehost for... 4 or 5 years and the script I used is no longer available.
If you wanted a tight reign on things, you could simply create a form and collect the info and set the account manually (giving out subdomains or using customer provided domains).