For the queries you have put across, let me start off with the strap-nets. These were rare, even in the days of the MUTTs & were installed mostly on Command-Cars like the one you have posted a picture of. A command-car was one used for communications. The nets installed were to prevent any loose item from falling off and breaking, such as a telephone/wireless handset.
As for you making one for your roof rack. I'm sure you have worked out something by now, as the post made was old. The best item you can use are nylon straps, stitched together. You can get these straps depending on your location from any place selling straps... If you are in Pindi, then try Raja Bazar. You can ask any cobler (mochi) to stitch the straps in the similar fashion. To ensure further strength, interwine the straps.
As for the M151, a lot has been said. It is an excellent piece of machinery. With a minor flaw that led to its sad demise & it never made it to the civilian hands (in the US market). The ones you see on the net are welded back items, mostly comprising of 2 or more different vehicles being welded.
Initially they were scrapped as a single piece, then came the cut; whereby they were cut in half and sold to scrap dealers. Then came the half cut & more; where each half was cut in an 'X' form. Each piece put in a different box & then discarded to scrap dealers. ensuring that no complete cut-off vehicle went to the same dealer. Even then when enthusiasts started welding these pieces back came the last blow. A 'cut & crush'. Cut vehicles were put under the crusher & the engine rolled over by a roller. Ensuring no one is capable of making it road worthy ........... all because of a minor 'flaw' ... termed as 'roll over at sudden turns'.
The M151 was prone to roll overs at high speeds & sudden movements because of its rear independent suspension. The earlier versions, i.e. M151 had many, then an improvement brought all the M151A1 & then again a further improvement brought the M151A2 (the version that came to Pakistan Army)
You can read about it further at my blog http://sites.google.com/site/g838paksq/
I hope you own one by now & if you do you know what a beauty it is, if you don't, you are missing out on it
PARTS availability will always be there, one just needs to be willing to spend (time and money), or you can always resort to a conversion.
Saqib Janjua (Janj) has a marvelous machine with great conversions.
Here's mine, still running on its original 2.4 litre gasoline engine.