Model Town used to do this simple, easy to assemble resin cast kit, for £20. I put this one together about six years ago. They did change the design a little while ago but sticking to the same idea, and a similar price. I think it may have gone up to £25. I just had a look on their website, but I could not see it, perhaps they have discontinued it, or there is another new one in the pipeline.
I bought one for a yard crane for Gernise End Station, on the Claptowte Railway. I constructed a platform for it, a 3mm ply box, with a scribed 3mm plasticard top, and, 1/24th scale textured brick paper walls. It is intended for indoor use, so would not be weatherproof, but the idea could be adapted.
David