This is an update of a post from a while ago, same info I posted way back when. May be helpful to you. Coincidentally, this cage is 24 X 48 also, except only 24” deep.
My advice is to invest in those corner clamps so you can get everything perfectly square when doing your assembly.
These are the 90 degree miter clamps I would recommend you invest in if you don’t have them- $3.65 each on Amazon

I also built a cage out of plywood and used the plastic track for a sliding glass front. I made an oak frame out of ¾ inch x 1.5 inch that was very strong and square so the rest would be very straight and square on the front side with the door.
I mounted another ¾ inch x three and a half board (poplar, another good straight wood) directly to the frame in front of that. The plastic track (which is exactly ¾ inch wide) sits on top of that board. I mounted a baseboard molding that came right up to the top edge of the track, so there was no way the track could be pushed out.
The illustration hopefully gives some details for you. The blue is the frame of oak, the green is the board the bottom track sits on, the baseboard molding goes on the front and covers the plywood sides on each end. The light yellow is the plywood. The floor is flush with the baseboard molding. The oak frame makes the cage strong and square and provides the overlap for the glass on each side and the top. The top track is mounted directly to the plywood ceiling.

Here’s a bad picture of this cage, but shows what it will look like. You can always modify to suit your build, of course.

Good luck, and post a follow up!