One of the disadvantages of developing for the new attiny´s  is that no native USB support is present, although UPDI interface is very convenient portability is a concern since you need to carry your flasher with you, in that regard I design a flasher that fits my needs perfectly and it has been extremely reliable.

The design is based on the CH340C chip that has up to day drivers and is cheep enough, the UPDI signal is feed to the D+ sign of the usb cable, so good quality shielded cables are recommended, but the flasher is extremely reliable using the SLOW or medium speed setting in megatinycore.

The main advantage is that you can upgrade or write and debug software in your project once is assembled with the inclusion of a USB .

Gerber file and schematic in project page.

https://hackaday.io/project/184703-gamelight-a-keychain-size-game-console-with-a-led