Use composite type to create new table
You can use a typed table:

CREATE TYPE mytype AS (some_id int, some_col text);

CREATE TABLE example OF mytype (PRIMARY KEY (some_id));

I added a PK constraint (which you did not ask for).

The syntax is documented as second variant in the manual for CREATE TABLE, which also explains further down:


  OF type_name
  
  Creates a typed table, which takes its structure from the specified composite type (name optionally schema-qualified). A typed
  table is tied to its type; for example the table will be dropped if
  the type is dropped (with DROP TYPE ... CASCADE).
  
  When a typed table is created, then the data types of the columns are determined by the underlying composite type and are not specified
  by the CREATE TABLE command. But the CREATE TABLE command can add
  defaults and constraints to the table and can specify storage
  parameters.


There is also a code example at the end of the examples section.