[harrysin]

All the 45° mitres on my many boxes were made on the router table. 22.5° router bits are readily available. It goes without saying that the opposite pieces in your case must be identical in length.

https://www.ebay.com.au/sch/i.html?...outer+bit.TRS0&_nkw=22.5°+router+bit&_sacat=0