I made two silnylon raincoats. We take them on backpacking trips where we don't expect rain, but need protection just in case -- same as your plan. We've used them occasionally for ~8 years and they've been great. On one Dirty Devil Utah trip we had ~4 days of very rainy 50 degree weather and they were fine. Sweaty, but not leaky or soggy inside.
I used a pattern that I got from OWF. I can find out the pattern name if you want. I found the pattern very helpful for making a hood that works well -- I had confidence about winging it for the body and sleeves, but I did want a hood that would work and I wasn't confident I could copy one from a raincoat.
I made the sleeves really long, so I can have my hands fully retracted with plenty of extra fabric hanging down, without having to scrunch up my arms and shoulders. I love it that way, and I've never found a commercial rain jacket that has really long sleeves.
And, instead of a zipper I just used narrow velcro, 1/4 or 3/8". I did that primarily because it was easy to avoid the nuances of a zipper. The velcro overlaps, so there's no need for a storm flap. The velcro is in 2-3" lengths, with 1-2" gaps between velcro. You don't need much velcro since there's no stress on it. Mine is not a pullover -- the opening runs the full length of the front, but I generally use it as a pullover and don't disconnect the velcro. I'm not sure there's any disadvantage to the full open front, unless you want to have a drawcord at the bottom.
Also, I didn't hem the bottom. The silnylon doesn't fray if you don't use it much.