In past special congressional elections in Texas,
neither a petition nor a filing fee has been needed. Texas filing fees are only for candidates running in a
partisan primary.  Therefore, fees don't exist in
special elections, since special elections in Texas
are non-partisan.

In 1961, because Texas posed no fee and no petition to
run in a special election, the number of candidates
for US Senate (to replace Lyndon Johnson, who had to
resign since he was the new vice-president) was 73 or
so.

In the 1996 non-partisan congressional elections in
the districts newly redrawn in Texas, there were also
lots of candidates in the first round, because of the
absense of any ballot barriers.  The 25th district had
eleven candidates.  The 30th had eight.  The others
had fewer because incumbents were running.

So I assume the same rules still apply, and neither a
fee nor a petition is needed for these 5 districts.