I sew strips of inner tube rubber to the velcro. The main reason is to protect the paint of the frame but the grip is firmer too.

The middle velcro strap of the under-tube bag is fastened with a screw and a shim in one of the water bottle nuts.

In the past I made some bags with straps but we (me and my bikepacking friends) discovered that they are not really necessary if the material is rigid, as the vinyl coated poly is. And that bag is never empty . I carry in it an ultralight rain jacket, a bandana, a spare tube, some tools, the pump and the medkit, and on longer trips another tube and the rain pants.

Here's the bike ready for a 3 days trip:


Total volume of the bags: 2075 cu in (34 liters) including the stuff sack
Total weight of the bags: 40,5 oz (1150g)
(My 35l backpack has 1230g)

The vinyl is on the heavy side, but is really strong, totally waterproof, easy to clean ant quite easy to work with. Having its own structure it stays where you put it

You don't need to hem the edges, just heat them with a lighter or you can do this if they start to fray.

I finally finished my bags. They're made of 18oz vinyl coated fabric.
Weight:
 framebag - 12.3oz
 seat pack - 13.4oz
 down tube bag - 5.29oz