Thanks for your reply.

You can tell that I am not an experienced MTBer as I had not thought about the getting behind the seat problem. I am a roadie with avulsed hamstrings so cannot ride my road bike for a while so thought a bit of pottering around on an MTB would be just the thing. Not planning on  doing anything steep and technical. Cannot afford any more injuries just now as also recovering from rotator cuff surgery.  Was just planning on getting off road and seeing the countryside rather than thrills and spills! 

Hi Everyone,

Thought I would like to have a go at a bit of bike packing and wanted to make my own seat bag.

Is there any particular reason that most seat bags are designed so that they run lengthwise, parallel to the rear wheel?

The fastest and simplest construction will be a straightforward holster, same as most peeps use on handlebars, to hold a dry bag 90 degrees to the rear wheel. I am guessing that this will remove the problems with tail wag etc.  Other than a possible aerodynamic disadvantage, which does not bother me as I am not racing, is there any other reason for running the seat bag parallel to the wheel?

All input from those with experience of different types of bags will be much appreciated.