fit 50mm Nikkor onto focomat IIa enlarger

[highpeak]

Guys, I just got a leitz focomat IIa and found the 50mm lens that come with it is not as sharp as I expected (go figure, it's a leitz lens!). So, I am thinking of put my new nikkor 50mm enlarger lens on it, but it's kind of shot to fit into the turret.

I was told to get a 15mm extention tube with 39mm male and female thread on each of the end, but I couldn't find it anywhere.

DO you guys have any suggestions how to put the nikkor lens onto the enlarger?

Thanks