Take a k-meleon.exe from a 1.1 install.
Rename it to k-meleon1.1.exe.
Copy k-meleon1.1.exe to 1.5 installation directory.
Start k-meleon1.1exe and use old Preferences dialog to write values
Alternative: type about:config into URL, go there, look for Cache and change Cache values.
p. s. 1.5 dialog will be repaired some day