The grammar of "Ploughman's lunch" is "lunch of a ploughman". So if you want two, you ask for "two lunches of a ploughman", ie two Ploughman's lunches (or even, "two ploughman's")

Grammatically, "harvestman spider" is a bit different - it's not "spider of a harvestman", it's "spider of "harvestman" type". So plural is "harvestman spiders". And I would consider that "harvestmen" is incorrect. But it's incorrect in the same way that "microwaves" is not the plural of "microwave oven" - it's incorrect because you are using the descriptor "harvestman" instead of the thing itself. But we do it all the time - eg "a bottle of red, please".