Just to make caron in US layout available not only on Ubuntu, but in Windows as well.
There are two options to use this layout:
1. Type caron AltGr+. separately before typing a letter for caron (čČřŘžŽňŇšŠ) and AltGr+0 for ring (ůŮÅå).
2. AltGr+z,n,e,r,c,u makes ž,ň,ě,ř,č,ů
Installation instructions:
1. Remove EN keyboard layout and input method before installation
2. You will need to reboot your machine after the installation.
Have fun.