I'm assuming you are playing 1.4.3 and to be honest I can't remember whether all of the temples in that version have temples. If I had to guess, I'd say yes, but its always possible one of the biomes got missed somehow as each one has its own special ruins..
Temples with portals always have passages into them with hedges that are controlled by hidden switches. They are always on the lowest level of the temple so you need to fight your way down. All the portals are already activated and this is why I suggest not making your own portal, as you will almost certainly be sent to one of them when you return which makes it really hard to get out again, but not impossible as I'm not so mean as to 'lock' someone in an otherwise 'locked' portal area. At least that's the case in the 1.50 version. Pretty sure its similar in the 1.4.3 version too.
If you find any 'hidden' buttons in a temple then you can be certain it has portal as they were only added in portal temples. The more I think about it the more I'm convinced all temples were given portals in the last update.
I'd try really exploring that nearby temple and look everywhere as some have semi hidden entries to the lower level.
To be honest, if do make your own portal, you may as well make it near your base. You probably won't be sent back to it when leaving the nether but at least you can get there using it.
One last thing, when you do come back, if it is to a portal in a temple, I'd suggest not disabling it as you'll likely be sent to a different temple portal the next time you come back after using your own portal. Fighting your way out of one temple is bad enough but not as bad as two. I've tried this myself to see what would happen and this was the result. No guarantees though as this game can do funny things.