De Nikon Optiphot uses 45mm filters at the lamphouse. These filters are quite hard to find at all and bl**dy expensive when they are offered. For special applications they still may be worth it, but I think a simple ND filter or a blue color filter don't need expensive technology.
I tried cutting filters from coloured gels and pressing them between two plexiglass disks. The great advantage of this approach is that any colour filter is available. The flipside, however, is that such sandwiches need be pressed together. The original filterholders are well equiped to do so, but the stack should be close to 3mm thick. Also, the plexiglass disks did not come clean from the laser-cutter. True glass disks (watch glasses) can be found on ebay, but I failed to find the proper diameter/thickness combination (45mm x 1.5mm). Custom made they are not that cheap, but perhaps it is still worthwhile if you frequently want to change the gel filter.
I am happier now with filter disks cut from Cokin P filters (such as this one
for as long as the listing lasts). The are made from coloured plexiglass and cut quite well. For a cleaner cut I leave the cellophane sleeves around it, but smudges can be also removed with IPA or glassex.
I get two 45mm disks and additionally two more 35mm disks from one cokin filter. They are but 1.8mm thick, but since they don't need to be sandwiched, they stand well in the filter house.
The colours available could be a bit restraining. Now I tried ND and blue. The blue is too intense just to cool down the halogene lamp.