Since no one has jumped in on this, i'll have a go at it.  Yes, you could use a rasp but you'll be doing a bunch of these.  That's a lot of rasp work and probably not a very neat job either.  If you have and extra hour, you could always make your own rabbet plane and use a chisle for the plane iron.  I did it using and old wooden molding plane for a pattern, worked great for what i needed.  Sorry, i don't have a picture but have a look at this link.  Same idea and should work for you. http://www.hometips.com/diy-how-to/tool-rabbet-plane-make.html

Ha, ok, I can now answer my own question and the answer is ... NO. You should not try to use a rasp or a file to create the gains. Having tried both, it is WAY easier to use the rabbet plane. The rabbet plane was easy to use -- especially by the last one. The only odd thing is that on half the gains, I had to plane the opposite direction. Mentally, it makes more sense to me to plane from shallower to deeper, so it was weird when this was reversed, but it would just chew up the wood if I didn't.

Cheers!