When I want a durable liner bag, I use a 3-mil contractor bag. The ones I have currently are "Husky Contractor Clean-up bags" that I think I got from one of the big box hardware store chains. These are definitely not perfumed, and hold up very very well. Downside, of course, is that they're heavier.

So I use them sometimes. For me, it's a flexible thing depending on the trip type and length.

Sometimes no liner and no pack cover, sometimes (very light, cuben) pack cover only, sometimes a thin/light/cheap yard waste bag if the trip is relatively short (definitely not durable) and not expecting a deluge, etc.

For my recent Florida trip I used the heavier contractor bag because (1) I was sometimes wading in swamp, to include once about waist deep, plus there was the infrequent steady hard rain that will defeat any pack cover, and
(2) for long distance 1-way trips (fly into one airport, fly home from another) the contractor bag is part of my checked-bag solution to keep my pack from getting torn up. I.e., have pocket knife and trekking poles so have to check baggage; put the pack inside the contractor bag with just the top exposed (so they can inspect, and so I can attach a name tag), using cord and duct tape to keep the bag in place. This has worked fine for me.

The contractor bags will last for months as a liner. They're relatively huge (too tall), or so they seem on first inspection --- and of course depending on your overall pack volume and volume of gear carried. But I don't trim them; it's great to have enough to be able to fold over on the top. My water bladder sits on top of the folded-over portion to help keep it sealed up. Last year in Florida I actually had to swim for a bit, so I took some time first to really ratchet things down, and using the contractor bag nothing got wet inside. Somewhat overkill for most weekend trips, however!

Some pack manufacturers sell custom made pack liners. I have no interest; a pack liner gets a lot of wear, will eventually develop holes, so IMO makes more sense to buy something cheap and local, off-the-shelf.