Personally, I don't think you *need* rules for creating new spells. Rules allow for cheese "But, it *says* I can do this in the rules!" Whereas lack of rules allow for the DM to say "No way am I letting that cheese in!" (not that the DM can't override the rules, you just hear a lot less whining about it if there's not a rule supporting your position).
Making a new spell is fairly simple.... there are a ton of guidelines right in the PHB - each spell is a guideline. Figure out what spells are closest to what you're trying to do, and use their level as a guideline, and fudge as necessary.
-The Souljourner