There was a recent thread on this (https://www.xs650.com/threads/valve-spring-compressor-recommendations.58574/). I posted a link to one on Amazon for $17.
To me...this is one of those things where the $17 for the tool is well worth the frustration of trying to do the job without it, or trying to make a DIY version. It's not exactly an heirloom tool, but for $17 I have no complaints. If you're careful it should hold up just fine, and you've got the piece of mind of owning the tool if you need to do the job again (as opposed to borrowing/renting one).
To me...this is one of those things where the $17 for the tool is well worth the frustration of trying to do the job without it, or trying to make a DIY version. It's not exactly an heirloom tool, but for $17 I have no complaints. If you're careful it should hold up just fine, and you've got the piece of mind of owning the tool if you need to do the job again (as opposed to borrowing/renting one).