Barry Railway:
For the 4 plank open conversion in 4mm it can also be converted from the Coopercraft kit. Both conversions need the axleboxes changed as well as brakegear. Photo on my website.
Diagram 57 cattle wagon can also be made from the Coopercraft kit with the axleboxes and brake gear changed. I'm not sure if the roof radius is correct as it's not right for the GWR version.
http://emardee.org.uk/ sell replacement ends for the GWR cattle wagon.
Diagram 60 10ton timber set can be made from the ABS GWR Mite Twin
Rhymney
The RR 4 plank loco coal wagon on page 104 of RR Drawings can be converted from a Slaters PO wagon.
The RR 5 plank open on page 112 is GWR built and can be built from an ABS O18 kit:

is also a GWR build and can be built from either the Coopercraft kit with the ends conversion kit or the ABS kit. Running numbers here:
http://penrhos.me.uk/RRVans.shtml
TVR

is a cross between a Slaters PO wagon kit and Coopercraft GWR wagon chassis with ABS brakegear. In TVR days it would have not had the GWR axlesboxes, a different metal chassis kit might be a better start point.