Originally Posted by
bikemig
The easiest fix is to get a derailleur with a claw.
...I'd have to agree. If you're using it as a trailer hauler, and cash outlay is a factor (which it almost always is), you should be able to slap just about anything with a claw hanger on there and have it work fine with your friction shifters. And if you don't have a large gear range on there (which I can't tell from your pictures or description), you might not need a long cage rear derailleur at all. This opens up your range of possible alternatives to a large universe of used rear derailleurs.
If there's a bike co-op within easy driving range of you, they'll have a bin of them on a shelf somewhere. Or you might be lucky enough to live near some guy who runs an old school local shop where they repair and sell old bikes. Those are mostly in college and university towns. I'd grab one for you from the co-op here, but by the time I buy it and mail it to you, you're better off going local if you can. And whatever I grabbed would probably not have a long cage, if that's what you need. those are less frequently encountered.