If the usage will only be to and from the vet, small animal (cat etc) could be used. Add a perch and you are all set. Try make it associated with more pleasant things,though, or your parrot will learn it's not such a good thing.
All I can relate is what we use (d). We started with a soft carrier that I brought him home in. In that short trip he managed to chew thru quite a bit of it. We got a pak-o-bird after that,meed size, and wee still use it for getting into and out the house, it gets used less and less, because SALTY is harness trained now, and he loves going on trips now. IMHO, that's the best way for your bird to travel, but it takes a lot of training to get to that level of comfort. See the threads on harness training for tips on that undertaking (took us a year of every night training to get there, but so well worthit). At 5 months it is not too early to start Treble training for a harness. For right now, a small wire type cage will do fine. Keep him warm, with lots of treats , maybe a few short practice rides before the long one to the vet, practice getting in and out of it. You want to keep the stress level as low as you can.