I use the hard case luggage rack with the extra Hide My Bell adapter that is likely made not exactly for this purpose but somehow matches the desired shape and serves the goal very well. I bought it "from the view". You only need a single hole to attach it:

It got a little bit angled for me but same way as the seat post is so the light sits even better.

Attaching such a thing to the bag from the flexible material like your seat post would be more challenging but it might work if to put some large enough plate with the hole on the inner side. You may also find some other place to screw it on.

The comparable part should not be very difficult to make even in a simple workshop, by attaching a L-shape elbow to something like thin plastic bottle of the suitable diameter.

I use the hard case luggage rack with the extra Hide My Bell adapter that is likely made not exactly for this purpose but somehow matches the desired shape and serves the goal very well. I bought it "from the view", was last in the shop under heavy discount. You only need to drill single hole to attach it:

It got a little bit angled for me but same way as the seat post is so the light sits even better.

Attaching such a thing to the bag from the flexible material like your seat post would be more challenging but it might work if to put some large enough plate with the hole on the inner side. You may also find some other place to screw it on.

The comparable part should not be very difficult to make even in a simple workshop, by attaching a L-shape elbow to something like thin plastic bottle of the suitable diameter.