CSB Round Timer draws a clean M:SS countdown for the round. It is computed from mp_roundtime and the time the round actually started (with mp_freezetime added on so the count matches the real clock), refreshed twice a second. In the final 30 seconds the timer turns red to warn players, and the moment the bomb is planted it hides itself so the game's own C4 countdown owns the screen.
mp_roundtime + mp_freezetime.csb_round_timer_hud.sma, copy it to addons/amxmodx/plugins/ and add it to plugins.ini.