Isn't really a good one. I go with Speick cream on the road, if you want soaps the only options are really a stick or a soap with a smaller container like Kell's or something. Face lathering in all cases.
Bring the soap, skip the bowl and face lather.
Better yet, just rub the puck directly on your face until you have a thin coating of soap over everywhere and take a damp brush to it - instant lather.
Col. Conk makes a travel bowl that is a nifty screw-top plastic thingamajig with a full-sized puck of Conk stuck inside. You can lather on the puck or use it as a stick. I like mine. ymmv
Edited to add the screw-top is not big enough to bowl lather in but it is big enough to wet down a smallish travel brush in.
I recently found that some of my irritation has been caused by face lathering. So I switched to bowl lathering and on the road I palm lather. Not as fun or convenient as face lathering but works just as well.
I love that idea about milling some Mitchell's Wool Fat into a used Irish Moos container for travel. Kinda inconvenient to take the ceramic bowl on the road...
1) get a puck of inexpensive soap, like col. conk, mama bear's or van der hagen. put into microwave-safe bowl and nuke on medium for short durations until melted. pour contents into screw top plastic or metal tin of your choice to make a perfectly sized travel-soap
2) use a shave stick. the speick stick is very small and makes killer lather
+1 to the guys suggesting bring the soap, leave the bowl and face lather.
Unless you are asking about the bowl that the soap comes in, in which case - bring that bowl.
Yes, use travel as an excuse to try face lathering! You will discover the joys of feeling those fine bristles that you paid so much for against your face that much longer.
I'm writing this as I await takeoff on a 777, down in cargo is my milled Valobra ....it probably has more legroom than I do!
I have a plastic bowl I travel with, I used to take a puck of williams and just keep it wrapped up in a paper towel when in dopp kit. Since then I have switched to bringing a $5 tube of Bigelow(proraso) instead for convienence.