If you can't print via flatbed or screen print and cut on CNC or die press, or outsource the job entirely then you're probably going to end up regretting excepting a contract to mass produce these.
Not trying to poo poo your parade but after a quick google image search for "coroplast windshield
signs" I found a company selling the same image as you posted and they are charging $8.95 for a 24" x 12"
sign. I don't know if you are trying to compete with those prices but if you are doing these by hand at a "competitive" price you and your client will probably walk away from this deal frustrated.
If you can't outsource, don't have access to any of the aforementioned equipment but your client is willing to pay you an appropriate price per for these to be done by hand with vinyl, then I'd say set your vinyl up with a mark for the slot and have someone make you a die punch that you can smack with a hammer.
Otherwise I'd say find an outsource or walk away from this one.