Well, as for holding their shape...the smaller the better, especially with finer fabric like linen...
a heavy duty cotton canvas mail sack works great, but so will a feed sack or even an old chilled shop bag...
If the fabric is too thin, actually lining it with ticking or linen beefs it up...
I usually stitch them up inside out, then turn them...and try to make them out of one piece of fabric to avoid a flap seam if I can...to do that just play around with the material folding and pinning it up with straight pins until you get it just right, then stitch it up....
Several years back I did a story for Muzzleloader about making a canvas market hunters double pouch from one long strip of canvas...heres a link to ordering a back issue if you would like to
http://www.muzzleloadermagazine.com/index.php?main_page=product_info&cPath=1_10&products_id=24 ...that might be your best bet...I covered strap attachment, antiquing, and other details as I recall...
tc