That's what I did on later frames circa '90 using True Temper 'Velo' aero tubes. Pic below. Their seat tube was much larger than Air though, and not very flattened, just enough of a crease in back to plausibly call it "tear drop", but probably not actually very aero. Very good stiffness at the bottom bracket though, better than most road bikes, felt as stiff as a sprint bike — very much unlike frames made with the original Air set, which were very whippy.

I was able to use a 25.4" round post, which conveniently fit the front curve, so I only needed "packing" for the rear airspace. I made a fairly thick pusher plate with the exact radius to fit against the seatpost, and the sides shaved down to snugly fit the inside surfaces of the ST. The pusher plate was pushed in with two setscrews, in the braze-ons visible in the pic. Those are like H2O-cage bosses, except M6 instead of M5 thread, and inserted from the inside so the flange (the brim of the 'top hat') is pulling on the inside of the tube, better strangth that way.

This is so much better than those frames where a setscrew pushed directly on the seatpost, a terrible idea. The screw gouges up the seatpost and makes a divot, so the next time you try to adjust the post a little, the screw wants to fall back into its existing divot. Lame! I'm talking to you, Vitue 979 (later models) though some other idiots tried it too. Mine as described above tightened snugly and reliably, requiring low torque on the screws. No damage to the post ever.

The pusher plate is brazed to a top cover shaped to fit the outside dimensions of the tube, so you can't see the pusher plate in use. On lots of them I brazed this top cover to the top of the seat tube and filed the joint flush so it looked like one solid piece with a 1" round hole in it. But it looks like in the one in the pic, I didn't braze the top cover, so you should be able to lift the cover and its pusher-plate out of the frame. This is 35 years ago so I don't remember lots of details of individual bikes. But I know this one was raced by a national champion in duathlon, the run/bike race (no swim). I guess that was a thing?


Also of interest in this pic are the brass fillets at the frame joints, which I'm proud to say were not filed, that's the as-brazed shape. I did a lot of fillet-brazing back then, got decent at it. Small fillets brazed quickly and not filed is one of the strongest ways to make a frame. Bigger fillets weaken the frame from the extra amount of heat they take, and the slower cooling from all that thermal mass. And any filing, no matter how good you are with the file, will inevitably thin the steel at the edge of the fillet — worst place to thin the tube since it's already weakened in the heat-affected zone (HAZ). But I digress!

Since I have an Air seat tube, I could determine the widest round post you could use, but it's going to be annoyingly narrow, since the Air tube is so much smaller and more flattened than this True Temper. And I think the widest part of the tube is a bit further back, so you'll probably need some packing in front as well as in back of the post.

But this is a job I'm equipped to do, if someone has a vintage aero/TT frame with no matching post, I can probably fix you up. Frame might need a repaint (could be just local repaint) if it needs brazing to make it work, like top-hat braze-ons for a pusher plate. Whatever the existing seatpost-binding scheme is, say pinchbolt ears with a slit, will rpobably not work anymore with the new scheme, round post with packing shims.