I used to have a side gig doing video and photography work, and am comfortable comipling and editing my own showreels, so I can't comment about templates or websites.
After compiling several showreels, I learned some important tips:
- Determine if the purpose of the showreel is to showcase your music or to showcase your work/credits?
- If it's purely to showcase your music, then you might be better off creating individual videos for each composition and compiling them into a YouTube playlist. That way, you can point people to either the playlist or individual pieces, and they can choose to listen to as much as they want at their discretion.
- IMHO, the best use of a showreel is to highlight your credits. You're basically saying "look at all these clients who have successfully used my music." It's less about the music and more about establishing credibility.
- Keep the individual clips in the showreel short. Potential clients don't need to hear more than 10-20 seconds of a piece to understand what you're capable of. The actual music is less important than we think it is. It'll be far more meaningful to someone that you got cuts on The Walking Dead, This Is Us, and Stranger Things than listening to the music.
- The total running time of the showreel can be as long as you want, but know that most people won't watch/listen past 1-2 minutes, so frontload it with your most impressive stuff.
Good luck!