Bit string help, please

Hey PostgreSQL Gurus,

I am experimenting with PostgreSQL bit strings to see if they might help
with some performance issues I am having. I added a "bit varying"
column to one of my tables. I have a PL/pgSQL function with an insert
statement into this table. For the bit varying column I would like to
insert a bit string of length n with the highest order bit being 1 and
all the other bits being 0. Given a table definition:

create table table1 (
a int,
b bit varying
);

I hoped I could use the feature where casting right pads 0s to do
something like this:

insert into table1 values( DEFAULT, B'1'::bit( n ) );

Where n is one of the parameters to the PL/pgSQL function, but that
doesn't work. PostgreSQL doesn't like having a variable for the bit
string length. Does anyone have any ideas how I could achieve this?

Thanks,
Ed Tyrrill