You can grab a link from a post on Instagram.

I have always just hand mitered my chainstays. No templates. I'm building a fixture because I want to check clearances ahead of time.

Paragon has tubing blocks. They said all of the sizes will be back in stock this week. Alex Meade has tubing blocks with oval cutouts for fork blades. I haven't seen tubing blocks for oval chainstays. Vee blocks work fairly well since there are a number of different chainstay sections.