I got a used handelbar stem, cut a piece of aluminum handlebar, and bought and mounted a used (ebay) Profile brand bike aerobar, though not sure which model. There’s a few similar ones, I think.
The stem fits on the part of the frame which holds the seat post, and some stems I tried didn’t leave enough room for the Radial’s seatpost quick-release clamp on the frame, but others did.
I first tried the stem slanting upwards and there’s pretty much thigh rubbing against the ends of the cut-off handlebar/aerobar clamps setup, which are a hair wider than the thinnest portion of the seat. Then I turned the stem slanting down and there was much less rubbing. I’m hoping I can solve this all the way with a shorter stem, so the handlebar/clamp is closer to the seatpost and hopefully far enough under my butt not to touch my thighs
I hope I can get it not to rub, because it’s very comfortable, similar, in this configuration, to a GB4 handle I tried (thanks Podzol!).
It’s lightweight, and easy to adjust and experiment with. The Profile aerobar can be adjusted lengthwise, and the handlebar can turn in the stem’s clamp to tilt the angle of the aerobar up or down. Also, the aerobar clamps can be rotated to be on either side of the bar, so the aerobar can clamp over or under the handlebar.
I tried the bar pointy end up, which is how bikers use it, but I like the GB4ish setup better. Here’s the current setup: