Short answer: Don't do it.
Redesign your app so that it can run in both portrait and landscape mode. There is no such thing as a UI that can't be designed to work in both portrait and landscape; only lazy or unimaginative developers.
The reason why is rather simple. You want your app to be usable by as wide an audience as possible on as many different devices as possible. By forcing a particular screen orientation, you prevent your app from running (usably) on devices that don't support that orientation and you frustrate and alienate potential customers who prefer a different orientation.
Example: You design your app to force portrait mode. A customer downloads the app on a 2-in-1 device which they use predominantly in landscape mode.
Consequence 1: Your app is unusable, or your customer is forced to undock their device, rotate it, and use it in an orientation that is not familiar or comfortable for them.
Consequence 2: The customer gets frustrated by your app's non-intuitive design and finds an alternative or ditches the app entirely.
I'm fighting with this with an app right now and as a consumer and a developer, I hate it. As useful as the app is, as fantastic as the features are that it offers, I absolutely hate the app because it forces me to use an orientation that is counter to every other way that I use my device.
You don't want your customers to hate your app.
I know this doesn't directly answer the question, so I want to explain it in a little more detail for those who are curious.
There is a tendency for developers to be really good at writing code and really terrible at design. This question, though it sounds like a code question and the asker certainly feels like it's a code question, is really a design question.
The question really is "Should I lock the screen orientation in my app?" The asker chose to design the UI to function and look good only in portrait mode. I suspect it was to save development time or because the app's workflow is particularly conducive to a portrait layout (common for mobile games). But those reasons neglect all the real important factors that motivate proper design.
Customer engagement - you want your customers to feel pulled into your app, not pushed out of it. The app should transition smoothly from whatever your customer was doing prior to opening your app. (This is the reason most platforms have consistent design principles, so most apps look more or less alike though they don't have to.)
Customer response - you want your customers to react positively to your app. They should enjoy using it. Even if it's a payroll app for work, it should be a pleasure for them to open it and clock in. The app should save your customers time and reduce frustration over alternatives. (Apps that annoy users build resentment against your app which grows into resentment against your brand.)
Customer conversion - you want your customers to be able to quickly and easily move from browsing to interacting. This is the ultimate goal of any app, to convert impressions into revenue. (Apps that don't generate revenue are a waste of your time to build, from a business perspective.)
A poorly designed UI reduces customer engagement and response which ultimately results in lower revenue. In a mobile-centric world (and particularly on the subject of portrait/landscape display modes), this explains why responsive web design is such a big deal. Walmart Canada introduced responsive design on their website in November 2013 and saw a 20% increase in customer conversion. O'Neill Clothing implemented responsive web design and revenue from customers using iOS devices increased 101.25%, and 591.42% from customers using Android devices.
There is also a tendency for developers to focus intently on implementing a particular solution (such as locking display orientation), and most of the developers on this site will be all too glad to help implement that solution, without questioning whether that is even the best solution to the problem.
Locking your screen orientation is the UI design equivalent of implementing a do-while loop. Are you really sure you want to do it that way, or is there a better alternative?
Don't force your app into a single display mode. Invest the extra time and effort to make it responsive.