i was about to cut a filter to fit but then i saw something here in RFF and gave it a go since i have the said gel at-hand from my days shooting flash
it saved me some time but it wasnt as nice as the one that I saw using a glass filter.
the real reason why the leica and contax patches are so sharp and contrasty is because they use a piece of "relay prism" to transmit light between the mirror and the splitter. on the contax, its a whole long shard and that made it exceptional. the leica m cameras have a short segment but it helped.
the reason why some Nikons had a feathered edge on the patch is more due to the mask (or the lack of it) on the receiving end of the beam if i am correct. you can improvise by making a square patch mask but that would make the patch smaller. this is probably the reason why the mask was omitted or intentionally made bigger. they settled with a feathered edge but giving you a bigger patch. notice that leicas patches are rather smallish compared to what nikon has.
on the flip side, a canon 7's patch is sharp because it has the mask but the resulting reflection is poor because it only has a dinky mirror and not a prism on the other size of the rangefinder
i thought that it was an awesome camera but i cant call it a "classic camera" because it was built like a toy. writing an overhaul article for it next year