Any camera repairman, many camera stores and any machinist can mill the
hole to the larger size.

or you can buy, rent, borrow a Greenlee Chasis Punch of the correct size
and do it yourself.

 Buy a blank board and have it drilled. But a blank board will cost more
then a used lens might. If all you want to do is test the lens maybe make a
board out of cardboard? Or something else cheap.

     Nick

    What are the focal lengths and which Beseler do you have ?

Regards,

  John S. Douglas, Photographer -  http://www.darkroompro.com
             Please remove the "_" when replying via email

  Which lenses do you have?  Do you have the retaining
rings for the back threads?
  I don't know Bessler enlargers well having mostly used
other types. For many enlargers or cameras the quick and
dirty way of mounting a lens is to make a cardboard adaptor,
they are surprisingly strong.
  Post 1946 Kodak made two clases of enlarging lenses,
Enlarging Ektars and Enlarging Ektanons. Both are good
lenses. The Enlarging Ektars were made for color work and
are very well corrected for color. The 50mm and 75mm lenses
are five element Heliar types, the 100mm is a four element
air spaced Dialyte. The Enlarging Ektanons  are not so
perfectly corrected for lateral color but are still very
good lenses. The 10" f/8 Ektanon is the exception, this is a
very high quality lens made for use on the Eastman autofocus
8x10 printer.
  Unfortunately, Kodak does not include mounting
information for enlarging lenses in its old lens booklets.