Hi Pauldelft The bags I have developed are way ahead of the steel pony and andy strapz stuff- which is a very good product but I have been told not so good for our northern Europe climate - the canvas they use is oiled and can sweat and become brittle in the cold. Its fine for the aussie climate where they were made and developed. the curtain side canvas like ortlieb stuff is very hard to work with if you want a waterproof product - the stitch holes where you attach straps and sew seams will leak. Most adhesives will fail due to the nature of the material and without very technical heat moulding equipment they can not be sealed properly. You can put on flat patches with a weld kit but these are not suitable for complex curves or repeated folding of the fabric. Ortlieb make the bags on tooling and pre set moulded presses that can cost upward of 100,000 euro to set up and buy. - really for the big boys eh. I am however sure you can make some panniers up - maybe for 50 euro I think a bit more but you do not build in your labour costs -and I would question the integrity if they were sale to others and not just for your own use. Also you will find the Buckles and fixings are all quite expensive if you were to use marine grade stainless steel If you contact a sailmaker I am sure you could buy some suitable boat cover PVC for your project quite cheap - I buy it for £6.00 a meter. However it has some disadvantages for pannier making also. It is not possible for me to go into all the various strengths and weaknesses of various materials on this page but I can assure you if you start talking to the various manufacturers of materials get samples sew various seams you will quickly start to learn about the problems. Good luck with your project I wish you well and hope you produce a bag set that is suitable for your use.
If I was inclined to do so I believe I could make a set of panniers far cheaper than I am - but then there would be compromise in many areas. Plenty other people sell panniers with compromise - that is something I am trying to avoid.