MySQL variable format for a "NOT IN" list of values

Going crazy trying to set a variable in a query of type:

SET @idcamposexcluidos='817,803,495';

so i can then use it on a

WHERE id_campo not in (@idcamposexcluidos)

I've tried defining the variable in different formats with no luck and don't seem to find an specific example for the above:

SET @idcamposexcluidos='(817,803,495)';
...
WHERE id_campo not in @idcamposexcluidos


SET @idcamposexcluidos=817,803,495;

with no success. It either returns an error or ignores the values.