Thanks everybody for your help.
I often print 16x20 and don't want to buy another easel. I already have a 4x5 enlarger, but wanted to use a second enlarger with the same set up and tools. Anyway, meanwhile, my genius husband solved the problem by replacing the bracket that holds the enlarger head to its frame. He used a sturdy piece of aluminum that he found at the local hardware store and bent it the same way as the original bracket, but made it several inches deeper. The head sits now more forward, and projects further than the middle of my big easel. He also replaced the screw used to control the forward tilt of the head, by a very long one. I was worry about the stability of the whole thing, but it is actually perfect. The enlarger is also bolted to my counter. It is all very sturdy, and after re-alignment, works perfectly. My two enlargers sit now side by side, and I just have to slide the easel from one to the other. So convenient!
So if somebody else wants to use a big easel with a 23C, that's the way to go!!!
Thanks bdial, jordanstarr, and eddie for your help.
Nathalie.