got a traditional sweatshirt kicking around the house? FWIW I picked up an inexpensive, thin, bright, orange one a cpl years ago for fall riding in the woods & it's quite useful. even fits under my helmet. if I get too warm I just take the hood off. think I got it at Walmart. would be better w/ a zipper, but this one is is so nice & thin. I have a thicker grey one w/ a zipper, that I sometimes use as well, but that hood is too thick to fit under my helmets
also have an inexpensive, thin rain jacket (not cycling specific) that falls into this category. but it doesn't vent well so I have to layer carefully
sometimes a vest is all you need. how about something with zip off arms, so you can regulate / vent more easily?