As promised (surprisingly) I managed to carve out a few more.
Trencher Officer and Sharpshooter Unit attachment. Looking at it now I think the sniper scope is a tad long, but hopefully it'll look better once painted.
Trencher Master Gunner. Blunderbus, knife and head from a GamesWorkshop Empire set.
And lastly the Trencher Rifle Grenadier.
All these figures came in at the spookily familiar price of 41 pence, totaling £1.64, instead of the retail of the official figures of £19.85. I think I've pretty much done the trenchers now with these plastics (barring repeats to take advantage of field allowance).
Any further proxying will have to come from a different figure source. And be cheaper than the official figure price.