Accessing certain diacritical vowel characters in the Spanish layout in Mac OS X I need the following Portuguese characters: ã, õ.

I write in Spanish/Portuguese all the time, but I am used to the Spanish keyboard layout. I can do anything but write the mentioned characters.

I do NOT want to constantly switch between Spanish/Portuguese layouts.

I would like to remap the key combinations: "option+a" and "option+o" to ã and õ respectively. Right now on those keys, I have these (å and ø) which are almost useless to me.

I cannot find a way to map those key combinations to my desired characters.

Can someone please help me? I downloaded keyremap4mac, but couldn't find a way to do it. Do you know a piece of software that would let me define the combination and the desired characters?
Option/alt + ñ on the Spanish ISO layout, followed by a or o gives you ã and õ.

To make a custom layout, use Ukelele

http://scripts.sil.org/ukelele

PS If you are running 10.7, you can also get those characters by holding down the a and o keys until the Character Picker popup menu appears.