Is there an easy way to add a custom downloaded jar to the classpath leiningen is using?

@retrogradeorbit install it in your maven repo. You want to do it anyway to make the build more reproducible

how do I install just a lone jar?

normally I'd have a maven project file and could go mvn install

but there is no project here, just a jar

you can still mvn install just a jar

you just have to tell it what coords to use

mvn install:install-file -Dfile=<path-to-file> -DgroupId=<group-id> -DartifactId=<artifact-id> -Dversion=<version> -Dpackaging=<packaging>