Millwall must build on "one hell of a performance" and look to claim their first win of the Sky Bet Championship season when they travel to Bristol City on Saturday, the Lions boss has said.
Harris' men suffered late heartbreak on Tuesday night as Ipswich Town stole a winner two-minutes from time and all three points at The Den.
The manager rued the "particularly frustrating" loss as The Lions dominated large periods of the match and once again had more than twice the number of shots on goal than their opponents.
"It wasn't the result we wanted, of course not, but I am really proud of the performance in so many ways," Harris told millwallfc.co.uk
"Any neutral would have been privileged to be at that match and plenty of Millwall fans stayed to clap us off at the end because they witnessed one hell of an effort - we just shot ourselves in the foot at the end.
"We will look to take all the momentum, all the positives, shape and structure that we have shown in our previous matches into the Bristol game. It's a chance to really relish and look forward to."
The Robins, who finished 17th in the Championship last season, started this campaign with a 3-1 win over Barnsley and will be hoping to climb higher up the table after breaking their transfer record for the second time in two years.
"This will be our toughest test of the season so far," Harris added.
"They are a side that have ambitions to be in the top ten this year, maybe higher. Bristol are a football club on the up and they have progressed so much over the last four or five years."
Millwall have no new injury concerns ahead of the trip west, while midfield recruit Ryan Tunnicliffe may make his first-team debut after playing 70 minutes for the U23s earlier this week.