The Thousand-Year Door has gathered less traction than its N64 counterpart, so there are lesser tools available for hacking the game. There are a few, though!

Wii.cs Tools

A suite of tools that can convert various image formats used by the Gamecube, .tpl being one of them. Said subtool is named TplMii.

Download Wii.cs here.

GCRebuilder

A tool that can be used to edit and rebuild Nintendo GameCube isos. Can extract and replace files, rebuild to a custom rom, as well as wipe garbage and change banner information.

Download the tool here.

Audacity (jackoalan's custom-fork)

A custom fork of Audacity maintained by jackoalan. It has the capability to open .stm, .dsp, as well as a few other proprietary audio formats. Can export saved work to .stm or .dsp as well.

Download it here.

xdelta UI

A tool that can be used to create patches for TTYD. Easy and not too complicated to use.

Download link here.