I have an old 20 Gauge Ithaca SKB 500, which is the 505 with a slightly different shape on the receiver -- the 500's design causes cracks in the walnut and the 505 fixes this -- and a longer trigger guard tang. Though I paid 500 bucks for it -- probably the cheapest good-quality O/U I'll ever find -- I still consider it a non-beater, and the cracks have been fixed with West Systems epoxy, the stock trimmed to prevent further problems. I use it for hunting, but I take care of it. Nice shotguns, those SKB's!
I bought a $50 used single shot with the intent of using it for Jeep duty, but it's one of those funky old Ithacas with a lever-action-style lever to break it open. It's real tight still, and though it has a lot of honest wear, it seems a shame to trash the old thing. Not worth much cash, but a classic in its own way, so I don't want to trash it.
Then I have this old 1100 I picked up cheap, but it's got the stronger Magnum receiver. Again, not valuable to a collector, but it's probably the best field-grade 1100 Remington ever made or ever will, so it's valuable to me for all-around use. And I have a shortened stock for it, too, so it's useful as a teaching gun, since it has no recoil with range loads and I can change up the length.
Just because a gun was cheap for what it is, that doesn't make me feel okay to use it as a beater, necessarily.
So, despite having bought a few good shooters for cheap, that leaves me with no real "beater" gun, except for the one shotgun I've ever bought brand-new, an 870 Express.
And that's what I'd recommend. The thing balances well enough for after-work trap league. 870s are anything but rare, so they're guilt-free tools, not museum pieces. Parts and accessories are available from all sorts of vendors, and they're still pretty cheap, even brand new. They're easy to clean, even in the field, with their modular construction, so mud and dirt are not your arch enemy. The newest ones even have a laminate wood stock, which is nice and durable for knockaround use, but still feels like a real shotgun when you shoulder it.
Others prefer the Mossberg 500. That works, too. The 870 and 500 are refined mass-production guns after all these years, in that they shoot well, work well, and their manufacturers can build and sell them for pretty cheap. A single shot can work, too, but it's sure nice to have a repeater, and these guns handle much better than the light single-shots IMO.
Or, just get a brand-new Beretta Silver Pigeon V and throw that in your truck. People will either be really impressed, or really appalled, or maybe a little of both.