This is what you must purchase. Mazda part #9954-10-1601, or an equivilant. Be sure to get two!
From mazdatrix:
10-1601-9965.....O-RING OIL FILTER.....TOWER ALL.......$3.30

Hmm.. Well, it sorta looks like the outer diameter is roughly 13/16".. Don't take my word for it tho. The inner diameter really doesn't matter TOO much, as there's quite a bit of room, as is shown below, as long as it's not rediculously small so that it gets in the path of the oil. The outer diameter is more important, as you want it to fit snugly into the pedestal recess.

Here we have the oil filter off. A good time to do this maintanence is when changing the filter--you will already have it off. There are two 10mm nuts that hold this piece onto the pedetsal. You must remove them both.

Here is the top of the pedestal. You can see the two bulges where the bolts are, and the bulge in the middle for the oil travel. Unfortunately, I didn't orient the pedestal as it is in the car...mentally rotate the picture about 55 degrees anticlockwise, and that's the orientation as if you were standing on the driver's side looking into the engine bay.

Here we have the bottom of the pedestal. You can see the threads that the nuts are held onto, as well as where the two o-rings matched up to it. When I removed it, the old o-rings were stuck to the base, still in the car. Clean this guy up a bit..

Here is a side-by-side comparison of the old (bottom) and new (top) o-rings. You can see how the old ones are very flat. They are also very brittle, and crack very easily.

Here I've put the new o-rings in the pedestal. Be sure to completely coat them in new oil first! They fit easily, but snugly.

This is the base of the pedestal, still in the car. My old o-rings were VERY stuck to this. The surface is actually flat (the o-rings do not sit in a recessed area on this), so I ended up using a razor blade to remove the old rings. Clean this area off, too. And really, DO NOT let anything fall into the holes!!! If you're worried about this, shove a wad of paper towel into it (not FAR..make it easy to remove), so nothing can slip in.

Re-attach the pedestal, making sure the o-rings don't come out. The tightening torque for these nuts is 6-8 lb*ft. Attach your new filter (you *DID* get a new filter, right?), and go ahead and change your oil....you're probably due anyway. Make sure you put the bolts for the pedestal through BOTH holes! I know it sounds obvious, but with such a cramped area, it's suprisingly easy to miss a hole completely. Start your car up, check for any leaks, and speed away!