I have a 2 blade Leica easel that will handle the 12x16 and a couple of the small 4 blade easels for 8x10 (cast alloy - the latest version - made sometime in the late 70's).
The 2 blade easel is good and it does have the slots for locking it down on a Focomat base board. This is nice as it will prevent the easel from shifting. They are heavy duty - built for production operations (newspapers, magazines, ad-papers were you printed to final images size - and needed to do it quickly).
My 12x16 is used on a Focomat IIc, mainly for 120 negatives (Rollei/Bessa III). For 11x14 prints from 35mm with the Focomat Ic I prefer a Saunders Pro-line 4 blaed easel on a 1" base. It is just as good as the large leica easel - and easier to find.
The Leica 12x16/16x20 4 bladed easel is even scarcer - but it is way to clumsy to use if you ever want to change image ratio - the blade are bit thin and tend to flex - and they go out of " square" easily.
On the Saunders 11x14 and 16x20 easels you can easily adjust and square up the 4 blades - and they tend to stay that way for a while.
If you check around on E-Bay and other auction sites (even Craigs List) - you might find something. If you are using a Focomat (Ic/IIc) - be sure to get the 1" base version. They are heavy enough to stay in place.
Another brand that I have used is Chicago Easels. Built like the proverbial brick - massive blades that would flatten even curly, heavy based 16x20 sheets and keep them flat. The 4 blades felt like they were made from truck-springs!!!!
Most of the larger easels made by Saunders. Leica and Chicago were intended for production facilities/graphic arts places and are usually "well used" to say the least.
The good thing is that a solid, well built easel is a one time investment - and will probably outlast you! It also makes the difference between getting increasingly frustrated in the darkroom or getting on with printing.
I tried the Kostiner easel some years ago - drove me nuts!! Nothing ever was square - the blades "flopped" around and would not stay in place.
The biggest problem with a big easel is simply the weight. They have to be heavy - the 1" base is not flimsy! Size is substantial and shipping a purchase could end up costing you more than the easel!
Also beware. if all you are printing is 11x14/12x16 it is fine - but if you are also printing a fair amount of 8x10 or even smaller - the bigger easel is a pain. You have postage stamp piece of paper on a large surface and the blades tangle up as you crop down.
In short, go for a 4 blade easel (more control) for the larger print sizes and also a good quality 2 - or preferably 4 blade for 8x10 sized prints. If you can find the small, heavy all metal Leica easel - it is a thing of beauty for 8x10 - and get a Saunders/Chicago for the larger size.
For some reason I remember that the small, all metal Leica 8x10 easel was listed in their catalogue in the late 70's for $700!!!!!!!. I suspect that I payed about $500 for mine and then I found another one at a swap-meet for about $50 (Bless the ignorants!) in the late 80's. Hence two - and both are used all the time for proofing.