Works flawlessly out of the box.
Downclock GPU to 200mhz
The game doesn't like any limitation of TDP, framerate suffers greatly. But downclocking GPU doesn't affect the framerate. I assigned camera controls (up/down, turn) and the save menu to the back buttons, otherwise the default layout works great. All-in-all, works perfectly.
Works perfectly
Found no issues
It's possible to enjoy this game through Proton, but...
The game works fine in fullscreen, until I alt-tab. When returning, it will stay in an unresizable window. Pressing alt+enter will make the window fullscreen, but only the upper-left 640x480 pixels of the screen will be rendered to, the rest is black.
Instead of running the Windows version of DOSBox through Proton, it's better to run this and other DOS games through the native Linux's DOSBox. Searching around, I found an unofficial compatibility tool for Steam that does just that (https://github.com/dreamer/boxtron). I Installed it directly from the AUR. There's no issue for me now.
Alt+Enter to switch to a window is broken, the window image is frozen. Also, if you Alt+Tab while fullscreen, the game gets windowed (and it works this time), but you can't make it fullscreen again.
I wasn't able to start the game at all
I've tried Proton 4.11-13 and 5.0-5 with no luck. I also had issues with MIDI that I solved (I needed a MIDI server) but that didn't help. The game just crashes even when it's run manually through DOSBox
Works without issues in Fullscreen. Alt+Enter to Windowed mode does not work, switching to windowed there is just a black screen.
Works very nicely
Perfect.
This is a DOS Box back-end, but they were too lazy to add Linux support. Runs perfect.