I use a small, purse sized, soft cooler (it's a backpack, actually, bought it to carry a few drinks when hiking) and ice packs. I bought it at walmart in the camping section for ~$5. I keep my insulin in the box, wrap the insulin in paper towel, and put it in a ziplock bag so if the ice packs leak it doesn't get all messed up. I also refrigerate (or freeze, if i have the room) the cooler overnight, otherwise it takes kinda awhile to cool down on the inside because it's more roomy than it looks. I have a lot of cooking-type digital thermometers ($15) so I will check the temperature of the cooler before I pack everything up. Keep the cooler in the cab part of the car (assuming you have a trunk).
I haven't had an issue, except for the one time (a few weeks ago, actually) that the cooler was in the fridge but the insulin wasn't in it and my DH grabbed the pack but never put the insulin in... what a surprise on the other end that was.. (we called our vets office in the morning and had them call a prescription in to the nearest pharmacy).
Willy travels well too. 2.5hrs (for us, where we live) isn't terribly long but I can see your kitty getting stressed if she hasn't done it before. I ALWAYS put some sort of calm collar on him before we go. You can buy a lavender one at walmart for $7. Others may know of something better. This last time, we didn't have time to get one (it was an emergency) and he was upset for the first 45min in the car. Feliway spray says it's good for this too, but I have not done that, personally. I want to try.
For long trips I put Willy in a large soft sided crate. It's
this one that we bought for our Lhasa Apso who (we figured out) hates being crated. It is very open (mesh "windows") and he is very comfortable in it. He can look out/see what's going on, it has multiple "entrances" (top, side, front), it's foldable, and he can move around reasonably in it. I LOVE that I can see him easily. It has the little bag that attaches to the side so we can stick all his stuff in there and it stays with him (except insulin). He will not tolerate being in a small/enclosed carrier for more than like 15 minutes so using this has made a HUGE difference for his stress levels. I am not sure if the wire crates for dogs would be good or not, but I think they are a cheaper option. The only thing is, the bottom is very slippery so we have the foam stuff used to keep rugs from sliding around on the bottom of it. The last car ride he got sick and I could see the openness being bad for that.
FOR US: Our last trip did NOT seem to spike his BG, at least not immediately. However, we forgot his insulin so he did not get his PM shot and got his AM shot very late in the day. He did spike, but we don't know if it was a result of traveling or a result of forgetting his insulin missing a shot (and the second one being super late). WE were also extremely stressed out and he was exposed to a lot of craziness and that could have influenced him, too. If you look at my SS, you can see what happened in our case.
Our trip was roughly 4.5hrs in each direction. We let him out during our trip being he likes to sleep on me but ECID. We are very, very, VERY lucky because he's part Snorlax. He's super chill/relaxed and generally very adaptable. He loves everyone and likes to just observe situations (rather than hiding from them or rushing into them). I don't think MOST cats would handle things as well as he does so my situation is very atypical. My dog doesn't even do as well!
