I've got a Source Double-D pack. I've added two sustainment pouches to the sides, a canteen and IFAK to the top, and I'll probably be strapping a REI Flexlite chair to the bottom.

(Yes, I know it's a horrible name and not a traditional backpacking pack, but I've had very good luck with Source products and I got an incredible deal on it.)

Any suggestions on a way to keep it dry in the rain? With the sustainment pouches, it's much wider than most traditional packs; not sure if traditional rain covers will fit.

Anyone have an opinion on poncho's that are designed to cover a backpack?

How big is the pack? Most rain covers are kinda stretchy, I'd think REI would be the place to look

Tarps are a cheap DIY option that can double as a ground cloth.  Otherwise measure your pack and check the various pack covers found in camping stores like REI, EMS, et .

Trying to post images from a phone...

I need one too for my 80 liter sized backpack. Meh, I prefer a subdued color.

I've got a Gregory that I bought with my old Gregory pack many moons ago- looking at their website they still make pack covers (backpacks then accessories)- in fact many pack manufacturers make covers and they're typically just an elastic opening bag of sorts so find something similar to your pack and it should work.  Sea to Summit makes some including their ultra-sil and REI sells their version pretty inexpensive as well.    

Regardless of whether you have a cover or not you still need to pack important stuff in waterproof bags as the pack cover will be open for the straps and belt and in a heavy rain it will get wet.  I've upgraded since my Boy Scout days in the 70s and 80s to the lightweight siliconized bags but if you don't want to spend much money just pack your stuff in Ziploc bags or stuff it in a large enough trash bag.  Works fine and you can use the bag afterwards so no waste.  

What kind of pack cover do you want?

For mostly on-trail packing, I really like Dutchwaregear's pack cover.  It will fit up to 50 liters easy.  For a more expensive, lighter option is cuben-fiber covers from ZPacks.

Condor also makes a multicam one if you want to go that route: Condor multicam

There are also some decent British (desert and DPM camo) and older USGI pack covers on Ebay (many of them listed as Jeep tire covers as well).  Those are pretty heavy material.  I have some British (or Ausi) covers that are brightly covered on the inside to double as a VS17 signal panel.  I like them when I plan to do more remote/off-trail hiking.

Even with a pack cover, I strongly recommend you pick up a box of trash compactor bags and use one as a liner inside.  Most modern packs will shed water pretty good on the outside, but if you take a spill in a river or lake or your water bladder breaks, your important gear will get soaked.

If you really want to try something innovative, I'm going to try using The Packa for my next wet-weather backpacking trip.  You just put it on your pack like a pack cover and then pull out the hood, arms and rest of the jacket to seal up the whole thing.  Expensive, but far better coverage and better breathability than my current rain jacket (OR Helium II).  The only downside to this system is that you're married to your pack; it's really just a trail cover/jacket.  You can wear it without the pack, you just look like a deflated hunchback

For the price, the Dutchwaregear's cover is light and effective; old school camo if you want it; black or brown.  Probably the best deal.

ROCK6

Pack size with the sustainment pouches is roughly:

Rough measurements:
Height: 24" (from butt to neck)
Length: 24" (from elbow to elbow)
Width: 13" (from spine to...well, away from me)

That elbow-to-elbow measurement is what I thinks going to cause me problems. I've got a tent + fly + stakes + footprint split between the two sustainment pouches on the sides. Cook gear in one "D" pouch; water filtration and toiletrees in the other. Main pack is food, sleeping bag, tent poles, and some spare clothes.

It's going to end up a few inches taller since I'm adding tying and REI Flexlite to the bottom.

I could take a picture of my Kifaru EMR that has the attached Scout piggy-backed along with two side long pockets and another back pouch.  I think capacity is over 120 liters.  The biggest rain cover is the 120 liter Bergen cover and it's pretty large:

DPM Bergen Cover

It's heavier than other materials, but if you want something non-camo, you'll have to contact one of the cottage vendors to make it for you.  However, for the price, the Bergen cover is a good deal despite the weight penalty.

My Kifaru EMR is stuffed with gear...with the larger Bergen cover on, it measures:

28" tall
24" wide
18" deep

...and it still has room to stretch.  It's pretty large...

ROCK6

Buy a yard or 2 of silnylon in whatever color you want and some elastic shock cord, and have a tailor shop do the sewing. Seal the seams with a 3:1 mix of mineral spirits & silicone sealer and you're good.

Cool idea. I may end up that way.

Anyone ever use one of these?

https://www.amazon.com/dp/B00Q2GHF1I?tag=vglnk-c102-20