How to set maintanence page

From an SEO perspective it's best to use HTTP::respond with a 5xx response status code ie.

when LB_FAILED {
    HTTP::respond 502 content $sorry_page noserver Retry-After 300
    return
}

The content returned could be a simple page such as one of the ones in bboyjnr's link above, or you can get a full-blown page complete with embedded images, then gzip it, then b64encode it, and place in a datagroup dg_sorry_page with key "5xx", then access it as follows;-

when LB_FAILED {
    HTTP::respond 502 content [b64decode[class match -value "5xx" equals dg_sorry_page]] noserver Content-Encoding "gzip" Retry-After 300
    return
}