How to add same character repeatedly without dotimes?

Sometimes I want to add some character repeatedly like generating something depended on how many there are. For example, organizing a file name with sequence numbers which might be different length, the length of sequence number 0 is 1, but the length of sequence number 1000 is 4, I want to organize like SEQ-0000 and SEQ-1000.

I use dotimes to fix this distance of sequence number character's length like below.

? (defparameter x "X")
X
? x
"X"
? (dotimes (n 5) (setf x (concat x "0")))
NIL
? x
"X00000"
?

This can do what I want to do but this is come true by side effect, so it is not beautiful. I looked for the way and the following might be best.

? (loop for i from 0 to 2 collect "0")
("0" "0" "0")
? (list "file-" (loop for i from 0 to 2 collect "0"))
("file-" ("0" "0" "0"))
? (flatten (list "file-" (loop for i from 0 to 2 collect "0") "345"))
("file-" "0" "0" "0" "345")
? (with-output-to-string (s)
(dolist (e (flatten (list "file-" (loop for i from 0 to 2 collect "0") "1")))
  (princ e s)))
"file-0001"
? 

In this example, hard-coded how many 0 should be added. But, editing this value to be determined dynamically, this function would be better.