With the clean tarp picture I was able to estimate where the tie down points were on a tarp and as Colin pointed out in his follow up comments there are some about 4' in from the edge as well as around the perimeter. I drew up a tarp to HO and made a PDF file with three tarps (click on the link to download the PDF). I then then taped some white tissue paper to an A4 sheet of paper, crossed my fingers and printed three tarps on my laser printer. This worked well as can be seen in the following photo.
After cutting a tarp from the tissue paper I used a pin in a pin vise to put small holes at the black dots printed on the tarp. I then threaded some EZ Line through the holes and put a small drop of ACC (Super Glue) on the underside to hold the 'rope'. Twenty ropes later I was ready to give it all up!!!
I then used a number 76 drill and drilled through the sides of the S wagon to the underneath of the floor .
A piece of balsa wood was glued to the floor of the S wagon to replicate the load under the brand new tarp in the Weston Langford photo. The balsa was rounded slightly on the top edge.
I missed a photo here, but the next step was to thread the 'ropes' on one side of the tarp through the bottom holes in the wagon until it was shorter than needed (so it could be stretched later) and glued it to the underside of the floor. I used accelerator for all glueing and I can't emphasise enough how this speeds construction. It is tricky to get two ropes through the same hole but it can be done, the second rope coming from the higher up tie downs. I continued slipping the ropes through the holes, pulling them tight then glueing them to the floor and folding the tarp around the ends of the wagon. This is hard to explain so if you give it a go you will have to work out the best way to do this. I think that each wagon type will have a different challenge.
Once the ropes were all attached with the tarp folded around the S wagon I carefully coated the tissue with watered down PVA and allowed it to harden.
The last step was to carefully paint the tarp without painting over the NSWGR stencils. I used a light grey and followed it with powdered pastel colours. I am not too happy about the result but it is the first time I have tried to replicate the colour of a tarp.
If you look carefully at the top view of the S wagon you will just see the NSWGR stencils.
ADDENDUM TO PREVIOUS POST
It appears that I am in trouble with the detail police as James McInerney sent an email saying that I had the timber in the wagons pointing the wrong way for the direction of travel (see the video in the last post). It seems that the single central stack should be pointed towards the locomotive to lessen the chance that either of the two outer stacks would strike something, makes sense to me, thanks James.
Here is a photo from Colin Hussey showing timber carried this way.