Does anyone know about who produces a 20x24" contactprinting frame ?
For that size you need a vacuum frame.
A conventional design frame won't be able to put enough uniform pressure on the materials to assure really good contact. If the spring force holding the back on is 50lb then the pressure holding the materials together is only 1/10 of a lb per square inch. But the pressure won't be uniform because the glass will bow, so some places will have very little clamping force.
A 20x24 vacuum frame has a clamping force of 14 lbs per square inch and puts no force on the glass. The total force is 6,700 lbs and the pressure is uniform.
You shouldn't need to pay much for an old vacuum frame. They are a bit scarcer now than they were 10 years ago when all the print shops were throwing them in the dumpster.
Another thing to consider is a "plate maker".
As hobbyists are now the largest consumers of these products, the smaller units command premium prices on ebay & co..