I have a Beseler 23C II XL and this is how I worked around the problem of the larger easel (mine is 16x20, but I bet it will work for 20x24, too).
1. Use the back of a scrap print to mark the left and top borders to the width you desire; for example, 1".
2. Using the largest paper slot on the easel (top slot), reference your scrap print to the left most position with your border marks in the upper left.
3. Set the left and top blades to your border marks.
4. Reposition the scrap print so the marks are now at the bottom right while still referenced against the left most position in the same paper slot.
5. Set your right and bottom blades.
6. Under the safe light, position the easel and focus your image. Adjust the one blade, if needed.

Using the top slot should give you room to move the easel into the path of the projected image. You will have to be careful that the easel still fits flat on the baseboard, which isn't very wide. If needed, you can always prop up that portion of the easel that hangs over the side of the baseboard.