Used up some odd bits from the spares box.
They were supposed to be for a painting demo
at the school club, but weren't needed, so I've
just gone ahead & finished them.

I used to love battlemats, especially the neoprene kind. If you can get them to lay out flat they do make a really attractive play-space for your models, with no terrain-making skills required. I have really fond memories of my first battle-mat I bought back in 2016. It looked absolutely 'the business' in comparison to the green tablecloths and even the classic GW grass mat I used to use before.

However, I have grown to hate them in recent times, because of two reasons. Due to an ever-growing family, a few years ago I moved my "games workshop" from one of the bedrooms of our house up into the attic. Ever since then, I have found that my battle mats gather an extraordinary amount of dust (the attic is a dusty place, apparently), which sticks to the rubber underside and makes them look terrible, and even a brushing doesn't help.

Furthermore, since the move to the attic, every single battle mat I buy gets its neoprene underside SPLIT whenever it is folded. I'm not sure if that's because of the temperature changes in the attic, or some other reason, but either way, it's put me right off them.

Other than the neoprene-backed variety, and that thin PVC material (which I thought looked really tacky), I haven't tried another material. I briefly tried going back to green tablecloths after the neoprene-splitting disaster, but that was a mistake. I think the only thing for it (for me), going forward, would be to paint a sheet of wood. I have since moved away from "outdoors" battles, though, and have started playing dungeon-crawling games instead, so am much more interested these days in dungeon-tiles than mats.

Hold the parts together mechanically...you really only want one thing moving to minimise the chance of errors.

This is important (I assume the 'one thing' is you with the iron).  Along with all the excellent advice given, keeping everything stable (and well lit) will allow you to concentrate on developing/executing "the touch".

I forgot we were doing another round but have confirmed my address as next recipient! 

Thank you for the kind responses to the army thus far. Here are the latest additions. I've been repainting early 90s spearmen and axemen uniforms so they fit in with the scheme of the older 80s models, that and doing Midgard movement trays for them, which is what they are based on in these pics. I'm pleased with them. The army is coming along nicely!