[kypfer]

Yes, the Cokin sheet holder is probably a cheaper solution than a regular filter, but I'm wondering about lightleaks.

Leaks and other annoying reflections can be a nuisance with the sun in certain directions in relation to the camera. Rotating the holder so the filters slide in from the side, rather than top to bottom, will often solve the issue!
The clip-on hood is also a sensible option, but for the occasional problematic scenario I use a length of black card wrapped around the filter holder, held in place with an elastic band
Yes, it can be a "bit of a fiddle", but I'm usually working on a tripod, for this kind of work, so there's no real rush.
As with a lot of my photography, a budget solution leaves me change for more toys

[Reciprocity]

Leaks and other annoying reflections can be a nuisance with the sun in certain directions in relation to the camera. Rotating the holder so the filters slide in from the side, rather than top to bottom, will often solve the issue!
The clip-on hood is also a sensible option, but for the occasional problematic scenario I use a length of black card wrapped around the filter holder, held in place with an elastic band
Yes, it can be a "bit of a fiddle", but I'm usually working on a tripod, for this kind of work, so there's no real rush.
As with a lot of my photography, a budget solution leaves me change for more toys

Yes I understand, good idea.