One of Sea to Summit’s ultra sil nano tarp ponchos which are 265 cm x 145 cm and weigh 230 grams (A$79.90 July 2019) as well as being excellent rain gear also provide all the shelter you need particularly if paired with an ultralight bivy bag of some sort. The US$16.99 Tyvek one I posted about here comes to mind. It is all the shelter you need. Some other suggestion s for bivies to go with ponchos: https://www.theultralighthiker.com/2019/05/30/ultralight-bivy-bag/
I have spent many nights camped under just such simple shelters in all kinds of weather – actually usually without any bivy bag at all – usually with just a scrap of Polycro or a mylar space blanket for an ultralight ground sheet.
Like this on a trip down the Thomson River for example:
Of course I have made my own 185 gram Pocket Poncho Tent which provides superior shelter to a simple poncho, but if you are not handy with a needle and scissors, Sea to Summit’s offering will do nicely. The ultra sil nano poncho above is 8’6″ long as you can see. If you pitch it like this instead of the way they recommend it is very unlikely indeed you will ever get wet. The one I am lying in below is only 7′ long. You can even pitch a 50 gram space blanket like this with the addition of a bit of dental floss which has many other uses (clothing repairs for example, if you carry a needle. I always do. – or a handy length of string.
Pocket Poncho Tent
You can spend too much money on gear, and carry far too much.