I made a PoC extending the SaphiRose with polycro (untrimmed, ignore the extra length and depth). It was a bust: there’s no reliable way to attach as the material is too slick, and if sewed I think the plastic will tear over time. It also looks dorky, more cheap than DIY cool

Its possible to buy a donor poncho and sew up an extension with matching material, but after some measurements its not a straight seam, requiring 3 panels and it would add a lot of weight. I think that’s too much work and I’m not confident one donor poncho would serve up 3 pieces of the needed sizes without more patch work.
I found a good solution, a hack on the classic poncho bungee belt. I made a 0.7 oz belt that goes over the whole pack using 3mm bungee and a cord lock

You need to cut the belt ~ 1′ longer than needed when worn and lasso it over the pack from the top front (easy one person operation). It does need a stopper so it doesn’t slide down the back, that’s provided by an xmid in the stretch pouch in the picture

Tighten the bungee belt either below the hip belt pockets (shown) or up at the chest. Doesn’t need much pressure, feels good.

I’ll still use a pack liner but this should keep my bag mostly dry, especially the top, absorbent belts and shoulder straps.
Its much more breathable than any rain coat. I left it on for 30mins in the house over the pack, and didn’t feel the slightest warm even with a thick wool sweater. I took it off and tried an emergency poncho under the pack and was uncomfortably warm in about 5 mins.
I think we might have a winner for warm weather use. It’s not full coverage like a Packa but it has some advantages:
– can’t complain about the price: $25 for the poncho, <$5 for the belt parts
– weight with pouch 11.3oz (~ same as 15D Packa)
– packs down to 10″x6″ (half the volume of a Packa).
– it’s not sleeved like a Packa so I’ll have to tolerate some water down the arms but that also makes it more breathable
It also has a full zipper for venting like a Packa while a S2S sil nylon poncho has no zippers
They don’t rate the hydrostatic head, but going on Bill’s experience and the reviews I’m expecting waterproofness to be OK.
I have a couple trips booked for May after the snow melts and will give it a shot.
Below 40F, I’ll stick with a light rain coat for warmth