Upvote:3
The Multi-City option is always more expensive and one ways often are too. Since your planned stop is less than 24h that will count as a stop-over and you should be able to get a single ticket for this.
When I have done this, I usually search a few OTA such as Expedia, Kayak and SkyScanner and sort by total duration, this usually gets me quite a few choices with a long stopover. Try it out.
When this fails, I go directly to the airline website and see if they will let me construct that itinerary. In the event this fails too, I have done a phone booking and usually got the same price as with a short stop-over plus a $25-50 phone-booking fee. All in all you still save money compared to other options.
Something that I have never tried and might help is Matrix. If you can search it and get the flights you want by putting your constraints, then you can take the output fare construction and use it to book your flight either with a travel agent or the recent BookWithMatrix site.