Most NIR opens were, as you say, cut down wooden opens. They were fairly standard across the board. Most were NCC with a few GNR - same design basically. Get one of Leslie's and give it a lick of paint (or, indeed, brake dust and filth!).
If you want to scratch build, I don't know where you'd get a drawing - though many here might - but a few kits or bought standard wooden opens from one of the British lines would probably do too, unless you'd be a "rivet counter"!