I know Botach Tactical isn't the most popular company in the gun world, but I've been using these for several years and I like them:
Falcon Low Profile Rail Covers
They're the cheapest I've been able to find anywhere for ladder covers, especially in 18-slot length. The quality is fine - they're soft molded plastic, kinda hard to screw up. And at $5 each, I don't mind buying some extras so I can cut them up and try different configurations.
For installation, I've found that if you work them back and forth in your hands a few times, it really helps them to snap on easier. Or if all else fails, you could use a hair dryer on them to soften them up. I've never had to resort to such tactics though.