Find flights with a stopover of a specific length at an arbitrary city?

Question

I have three days to get from Singapore to Budapest, and I would like to find a city to stopover for two nights. I don't have a specific city in mind for the stopover. Are there any flight search engines that can search for such flights?

The best I know how to do now is enumerate all the big transfer hubs and entering it as a multi city itinerary in Google Flights, but that is very slow and I will probably miss out many reasonable cities.

More formally, given cities A and C, and dates D and D+N, how may I find flights from A to B on date D and from B to C on date D+N for some city B, preferably sorted by price or some other reasonable metric?

Apparently ITA Matrix used to be able to do this, but all the guides floating around the interwebs use some older version of ITA Matrix that has extra buttons not present in the new Google-y material design website.

Answer 1

There are two options of booking this: A) as a single itinerary with a stopover, or, B) as two independent tickets.

Option A) is dependent on the stopover rules of a specific ticket. These can be complicated, e.g.: none, one per direction, unlimited, various fees, etc. This doesn't lend itself easily to search engine integration and since demand for this is rather low, no engine I know has bothered trying to integrate it.

Even most airlines keep that somewhat opaque: Stopovers can only be booked as a multi-city ticket and a ticket from A to B to C could be issued as a "A->C with a stopover in B" or as two separate fares "A->B" + "A->C". This being said: Some airlines (e.g. https://www.cntraveler.com/story/airlines-with-free-stopovers) do actively advertise free stopovers in their hub.

Your best shot here is to do one of the following

1. Look for BUD->SIN, identify the top 5 choices that meet your criteria for attractiveness, detour, tolerable pricing, etc and manually check the stopover rules for each fare
2. Look for "extra long layovers" with ITA matrix. While MINCONNECT 36:00 does indeed come up empty, MICONNECT 24:00 or MINCONNECT 30:00 provides a good list of options. This can also be a great starting point for method 1) above.

An extra long layover may have the advantage that the airline may hold on to your checked luggage. For a stopover you will always have to collect and recheck bags.

Option B (two separate tickets) is a non-starter for a search engine. There are several hundred potential stopover airports each with dozens of flight options to/from BUD and SIN. The sheer number of possible combinations will be prohibitively large.

So you would have to constrain the search somehow. Both BUD and SIN serve 100+ destinations. If you want to minimize the amount of flying, you could look for destinations that are served by both airports and geographically desirable (by your preferences, visa, etc.). A good tool for something like this is flightconnections.com.