You didn't mention what exposure time you consider to be too short, but is 10-15 seconds about what you are seeing? Is the bulb designed for 120v and being run at 240v?
Of course, it is possible that you want a minute of exposure time in order to dodge precisely - maybe an alternative to the ND filters is to make a 20" print with all the adjustments you want and then to re-photograph it, making a more or less straight print of the copy-neg.
I bought a 0.9 ND wratten gel filter on ebay for $1 and simply laid it on top of the rear element of my enlarging lens. Works great. It is a large 300mm Componon-S, so the 3 inch filter works well, for a smaller lens I would trim it down to size.