Crosscutting wider panels to trim them down to size or to square them up can be a frustrating experience, unless you have something like a track saw set up especially for that task. Even then, you will be giving up space in your shop to a tool and setup that you may not need, if you have a table saw.
I’ve made table saw sleds that will do this operation with panels, but they were very limited as far as panel width goes. They are also bulky and take up a lot of space, making them less convenient to use and to store.
So, I gave it a lot of thought and came up with a fairly simple jig that is compact and lightweight, and also has the capacity for the widest panel I’d need to trim.

The first thing was to find a suitable scrap of plywood to make the base. I used 1/2″ thick Baltic birch that was 6″ wide that I cut down to 50″ long. I ripped the edge off to use as the guide bar that fits in the miter slots in my saw and fastened that to the edge with glue and pins:

At 50″ long, that provides a bit more than 36″ crosscut capacity, which I think is the practical limit for a stand alone jig like this.

The jig has two main parts: the base and the clamping bar. The clamping bar uses two toilet flange bolts that project up through the base, and here I’ve routed a recess in the bottom of the base for the head of the bolt that will slide back and forth to adjust for wider panels:

The bolt in the slot. The recess is just wide enough to allow the head of the bolt to slide, but will also stop it from turning when you tighten the clamping bar:

The fixed bolt at the other end is tightened in place with a nylon insert lock nut to keep it from turning:

Even with the 50″ base, you can get even more capacity by using a longer clamping bar and cutting the slots longer. As it is, I would be using mine for panels that are 16″ to 36″ in width.

The clamping bar doesn’t require a lot of force to hold the panel in place, so regular wing nuts will do the job nicely:

The first test cut went perfectly. The base of the jig as as a zero clearance and keeps the edge of the panel from chipping during the cut:

I use the jig very much like you would use a track saw, and just line up marks with the edge of the base before tightening the clamping bar.

To keep wider panels level, I added an interesting support to the edge of my table saw that slots down flush and out of the way when not needed:

Another less permanent way is to just use a strip of 1/2″ thick wood under the panel to hold it up. Shorter panels can be cut without the extra support.

As an after thought, I added a piece of 1/8″ steel to act as a squaring fence:

It can be adjusted to perfectly square by loosening the screws. I used 1/8″ steel because I want to be able to cut material as thin as 1/8″ thick with this jig, and didn’t want the fence holding up the clamping bar.

I made a video showing the build plus using it: