You have to do some real digging, but sometimes you can find a rubber straight bar snaffle that is narrower. I actually have one, but no idea where it came from!
Nathes can be a little less expensive and have a very similar feel.
That being said, why are you so sure he'll "mature" out of it? I LOVE horses that go in a Nathe or Duo. Toby has actually matured INTO a Nathe. And I started a little mare years ago (she's now 11. I had her from 3-6) that went in a variety of plastic and rubber bits before settling into a Duo at about 5. Her rider now rides her in the EXACT same set up I did...Duo with a plain cavesson....she moved up to prelim this spring.
So, anyway, don't be SO certain he'll grow out of it. In my ideal world they stay in them!
VTO sells a thin-ish version of a rubber bit with no metal running through it, just rubber. I don't know exactly how thick it is, but they also carry the Sprenger Duo Dee, and I'm sure their customer service folks would be willing to measure them both. http://www.vtosaddlery.com/Merchant2...Category_Code=
Otherwise, consider doing what people did before these new-fangled bits came along: they used leather bits. eponacowgirl here on COTH is a big fan of those, and she got some nice thin ones from Bedford Tack in Tennessee.
ETA I'm not saying I dislike the Duo or Nathe bits. They're great, and they're more durable than the older rubber and leather bits. But they're pricey, and the older technology is cheap enough that you could buy 5 rubber or leather bits for the price of one Duo/Nathe.
I was in the same situation two years ago when I got my Canter MidAtlantic cutie (Shoes). He loved the the D version of the Duo. I didn't want to spend the money for something he would "grow out of". Well, I finally decided that he was going through enough changes in his life (like a move half way across the country!) and he didn't need a bit change also. I sprung for the Duo and I am glad I did. He is still happily in it and most likely will be for a long time.
It was an expensive initial purchase, but knowing that it was what he liked and that it would not just end up gathering dust made the decision a little easier.
Alter- I think the loose ring version would be fine. I have ridden him in other bits but he really does prefer the duo. I have a few duo's and nathes in my bit collection. I think either works. I normally do the D-ring version with the greenies if they are wonky in their steering but he is not. It just happens to be the bit set up on the bridle that fits him.
I find the sensitive Tb's really like the feeling of the duo. I got hooked on them when a trainer suggest one for a horse that I had that feared the contact and would rear up if you touched his mouth. It totally transformed him and in the duo he was as soft as butter. I find that they will stretch down into it unlike any other bits so the horses that tend to get a bit braced/short in the neck really do well. If you have a horse that leans or is heavy then it isn't the bit for you.
Most of mine do not outgrow it and I think that is a great thing. I love a horse that is so soft and adjustable that I don't need anything more.
I bought mine in a tack shop for $75? The rings are not quite as loose as say a kk. But it's a great bit. We're not currently using it as horsey is doing well in Myler, but we did for a while and I'm never getting rid of it, it's awesome to have on hand.
My (not nearly as extensive as Jlee's) experience does suggest that TBs prefer D-rings (or eggbutts, but fixed) over loose rings. I've done the Nathe/Duo, The KK D-ring and on current boy after trying those, got much better stretch into the bit with the Myler comfort snaffle (started with a D, now in a Boucher). I'm guessing that while you said "ouch" when you hit the order button, you're going to be happy for a long time going forward. Looking forward to hearing about him as he settles in with you and makes his eventing debut.