I just got a West Loop Mini Backpack from my local store, and I added a Soft side Stroller bottle holder I bought off Amazon for my old Discraft Tournament single strap bag and used on the new backpack via heavy duty plastic Cable ties. I used some Clear Flex Seal Spray for the bottom of he backpack with a piece of cardboard inside to keep the botom full shape. This part is optional I also had an Outcast set of Backpack Fishing Straps for a duffle bag to be a bit more like a backpack.
This will be my Beginner do not play much bag for someone, all they need to do is bring the 25-30 oz water bottle fit is depending on the shape. I will put some older discs in the bag that I do not use that are beginner use able. I already put two plastic jar lids in for the mini markers that are within the legal dimensions for a mini marker.