Thanks guys!
@Graham: It's a very cool idea to have a set on a tree, but unfortunately 3D printing is (still) too costly to put any waste material in there for the sake of being cool
The way things are looking now, the sales price of the +8mm mounts will be 14-17GBP.
The current design is too large to be posted in a Jiffy Bag, so I'll also make a version with two parts that fits through the mail box - The 3D printing allows such flexibility to offer loads of options and versions, so why not make use of it?
As for colours, The material is white as standard, and can be dyed or painted. The dying ensures the material has the same colour throughout, but I have no experience with it (yet). Painting (only tested so far with Tamiya PS paint) doesn't go throughout the part, but adheres very well. With the white material, it's possible to take virtually any colour, including Fluorescent colours:
@Tom3012: Thanks for the suggestion
I have my concerns it'll compromise the strength of the mount. The spacer may also have a considerable amount of material in it, and as such may be costly on it's own. I'll look into it though, it's an interesting idea!