Salihomba, that's awesome. Never thought that!

jeroldharter, I don't open to maximum aperture in the fear of uneven light.

One thing that I haven't tried is to fit smaller diffusion chamber and try to use 2.8/50mm enlarging lens. It's two stop faster than my 6x9 lenses and perhaps smaller chamber also gives brighter light.

These enlargers are Meopta Magnifax 4 with multigrade diffuser head and condenser are cheap and lightweight Meopta Opemus 5 (only up to 6x6).
I have never before considered them slow or dim enlargers, especially Magnifax 4 gives too strong light for normal printing, I have to adjust light intensity to lower settings usually.