Actually, I've found an old Durst 805 6x9 enlarger. It lacked the condensers,
bulbs ... only the skeleton remained, with nice and sturdy column and baseboard.
The head was large enough to allow a hole to be made for a 4x5 neg carrier,
which I did.
It had no bellows but a large cylinder, at the end of which you put the lens,
and this cylinder screws into the head. Screwing or unscrewing moves the lens
nearer or further from the film plane, but it was not able to move far enough
for long focal lenses (135 or 150mm) to be used. I then made a bellows to be
placed between the cylinder and the head.
The main issue was, as always, the light source. As I don't have any condenser
nor the ability to build one, I first wanted to buy a 4x5 head, but it proved
rare and expensive, so I planned to put a garden halogen lamp over the head,
and diffuse the light with some ground glass. I placed aluminium sheet on the
inside sides of the head to reflect the light, but though there was a big fall-off
in the corners. I then changed the light source : I drilled 5 holes in a small
board, in which I placed five 60 W krypton bulbs. Four are placed in the corners,
the fifth in the center. Between the neg and these bulbs, I placed what I have
found to be the best diffuser screen, both for the homogeneity of the diffusion
and the little quantity of light it absorbs : a simple sheet of tracing paper.
The last problem was that it was far too hot with 300 W, and some plastic parts were melting. Hence
I placed two
computer fans : one vertical behind the neg carrier and the tracing paper which
blows air inside the head, and one horizontal over the bulbs, which pumps the
air from the head to the outside. It perfectly works : the bulbs can stay on as
long as I want.

It's nearly over : I thought it was but it remains to seal all the light leaks,
and to print the first tests! So far, I've only one concern : the fans induce
some vibrations, though they were fastened with flexible materials. It's hardly
noticeable, but I fear it might affect the sharpness of the print. I'll have to try, probably in a few days, to know.

I probably will post some photos and comment the results in a thread if all goes well...