Given the amount of plastic in one of those Nikon holders, I'd expect the OP might be better off just paying the ridiculous ebay prices. Something that large and dense could easily run $200 without an aggressive post-design material minimization.

In traditional injection-molding, the material costs quite little in comparison to the cost of making the mold. But in 3D printing, the amount of and type of material used is the primary driver of cost.

If the OP finds a source for these, buy me one and I'll pay you a fair price...seriously.