I had a dog and a cat in my van for seven months. I trained Trilby to a leash, had a litter box, no problems. But I can tell you right now, that if you think you're going to teach your kit to a leash all in one jump, you're dead wrong. You'll see people on YouTube dragging their cat. STUPID. Get one of those wrap-around harnesses, gradually get her used to that. Clip a leash to it, and let her drag it around. Pick up the leash and FOLLOW her around, w/no tension on the leash. Gradually get her to walk with you with slight tugs. Patience is the name of the game. I seriously doubt if the there is a harness that a cat can't get out of, if she's panicked enough. DO NOT EVER let it get to that point.
Do the same with riding in the van: Baby Steps. See that she's comfortable with each step before moving on with the next. Put her in the van, with the van just sitting there. Feed her in it. Read a book in it and let her explore it. Later, turn the van on and just let it idle with her in it. When she acts normal with that, drive around the block and then stop it. Be sure YOU act normal, and don't 'poor baby' her into thinking something is wrong.
A folding wire dog crate is good, one like this:
https://www.chewy.com/frisco-fold-c...GHfTWlHIXpTxgWtkJ2xoC3iAQAvD_BwE&gclsrc=aw.ds My crate is 36" long, and folds to 36x24x6". Don't get one with wheels, as they take up a lot of unnecessary space.
A larger crate will hold a litter box, a bed and water (they usually come with a floor tray). It will keep you from losing her if you get distracted and leave a window or door open, if you're in an accident, and when you arrive in your new camp and are setting things up. Use a plastic tote for a litter box, and cut a hole in it HIGH; if you place it low, she will push the cat litter out. Male cats will sometimes do a half-crouch pee, and go over the edge of a regular litter box. You sure don't want that! If you get a kitten, use a smaller tote & cut a lower hole, and replace it with a larger one when she gets more cat-sized.
Get or make van window screens. You can buy screens that attach with Velcro, but IMO, Velcro and cats don't mix well. All you need is for some moron to punch the screen as a "prank" (God, I hate that word!) and your cat is gone.
What I did then is what I will be doing again: getting some cheap, thin wood paneling (used for walls) and making framed screens. Make a paper or cardboard pattern of the actual window opening, and then
cut off one inch all along the bottom. Inside that pattern, draw the same shape, but 2" smaller all around to make the hole for the screen. Trace onto the wood (one right, flip the pattern over for the left -- I put the colored side facing outward). Cut out both edges with a jigsaw. Draw a line on the inside of each bottom, 1/2" up from the edge, and about 1" in from each side.
Get one 3- or 4-foot strip of 1/4" h-channel aluminum shaped like this:
h (maybe $5); it also comes in a 'capital H', but you don't want that. A j-channel will do in a pinch, but I think the h gives more support and security. Most decent hardware stores will have this. Cut each strip the width of the bottom of the window MINUS about 2". Use a hot-melt glue gun (low-temp melt won't work) to run a wavy line above your drawn line and set the bottom edge of the strip of aluminum on the line with the open h-channel facing DOWN (the channel will catch on the edge of the glass window) and centering it at the ends. Press the channel down firmly into the glue and let set until cold.
Get about 3 feet of ALUMINUM window screening (nylon is too weak --
do not use). Cut it to fit about 1/2" inside the edge of the wood frame, and just above the h-channel strip. (If you make it the same width as the frame, you will cut your hands on the raw wire every time you use it.) Using your hot-melt glue gun, run a fairly thick wavy line of glue just above the h-channel, center the wire mesh on and just above it, and press down with popsicle sticks or something similar, adding more hot glue on the top so the glue oozes through the mesh and connects on top for a tight bond. When cold, apply the glue to the other three sides of the screening. Then go over the rough metal edges of the screening with your hot glue and popsicle stick, covering the rough edges of the screen.
To put in place, roll up the window glass about one inch, set the aluminum channel over the edge, hold in place while you gently roll up the window, fitting the top of the wood frame into the upper slot where the glass usually goes.
p.s. I would also have a soft carrier for trips to vet, etc. Soft carriers can be folded and squashed to fit in spaces that rigid carriers can't.
