Here is a photo of my home made filter holder.
Basic part was just a rubber plumbing fitting - cost was about $15.
I then just cut it down a bit because it was too long.
A thin sheet of aluminium was then screwed to the rubber fitting after a suitable hole was cut using a jig saw.
I cut some plastic to create a suitable groove for the filters to slide into. This was lined with some felt my wife had - hence the white rather than a black felt.
The plastic filter holders were then screwed onto the aluminium sheet. Job complete.
I bought some glue to attach the felt to the plastic, a few screws and the plumbing fitting. Total cost $30.
The filters being used are Hitech 165mmx200mm jobs - and they cost ten times the cost of the filter holder.
Hope this inspires you to save some bucks by making bits you can make because I sure as hell can't manufacture my own gradient filters.