A quick look at the slate wagons kits available in 16mm/ft at the moment (Autumn 2021)…
‘uss_Raven74656’ on ebay – £8 – https://www.ebay.co.uk/itm/194296426551 3D printed model of the steel slate wagon
The Lineside hut – £9 – https://www.thelinesidehut.co.uk/quarry-slate-wagons.html Laser cut plywood 3-plank
(A resin slate load is also avaialble for £5)
PDF models £9 – https://www.pdf-models.co.uk/product-page/slate-wagons Plywood 2-plank
(bulk discounts available)
Binnie Eng (Approx £13, but not currently available) Colin and Peter’s original 3-plank wagon.
Peter’s FAQ says that he’s in the process of making new moulds – https://peterbinnie.com/faq
How Colin Binnie made the originals https://www.colinbinnie.com/slate-wagon.html
Resurgam £18-22, MDF or Plywood – FR 2-ton steel waggon
https://resurgamrollingstock.co.uk/product-category/rolling-stock-kits/ffestiniog-welsh-highland-railways/slate-waggons/
IP Engineering – £20 – https://www.ipenginnering.com/product-page/ffestiniog-slate-wagon Laser cut plywood 3-plank.
(Note: Search engines may give you this page http://www.ipengineering.co.uk/page120.html, listing the Binnie wagon at £13. This page is quite out of date and these wagons are not available)
Slaters – £20 – £26 – https://slatersplastikard.com/wagons/16NGWagons.php – Dinorwic or Festiniog wagons, may contain brass etches.
Bole Laser – £30/£35 – https://bolelasercraft.com/product/talyllyn-railway-slate-wagon/ 3D printed 2-bar as on Tallyllyn
Coast Line Models – from £17 – https://coastlinemodels.co.uk/ various kits for steel and timber slate wagons (requires Slaters’ wheels to complete)
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This article explains how to make your own: https://www.16mm.org.uk/2018/09/01/september-2018-ffestiniog-railway-2-ton-slate-wagons/