If you're set on using a miter joint, you could do a splined miter. This is a miter with a captured spline to add rigidity and strength of the long grain glued connection as opposed to the splines going across the outside of the miter. This excerpt from Woodcraft shows the type of spline I mean, it's something you should be able to achieve with a saw or small enough router bit, but make sure you have a fence setup with that to keep it stable and you safe during the cut.

https://www.woodcraft.com/blog_entries/splined-miters