Tips for re-using function results within single insert

using this table:
create table test_table (a text, b text);

This would be nice to be able to do...

insert into test_table (a,b) select random()::text as "myrandom",
encode("myrandom",'base64');

Any ideas on how to accomplish this without

create table test_table (id serial, a text, b text);

insert into test_table (a) values (random()::text);
update test_table set b=encode(a,'base64') where id = currval('id_seq');

CG

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