ADDED   Set3000x2000Res.m
Index: Set3000x2000Res.m
==================================================================
--- Set3000x2000Res.m
+++ Set3000x2000Res.m
@@ -0,0 +1,57 @@
+/*
+ * This small program sets the resolution to 3000x2000 in HiDPI mode.
+ *
+ * Unfortunately, this is necessary because macOS does not allow selecting 3:2
+ * resolutions as used by some travel displays.
+ *
+ * Compile with:
+ *   clang -framework CoreGraphics -framework Foundation Set3000x2000Res.m
+ */
+
+#import <CoreGraphics/CoreGraphics.h>
+#import <Foundation/Foundation.h>
+
+int
+main()
+{
+	NSArray *allModes = (NSArray *)CGDisplayCopyAllDisplayModes(
+	    CGMainDisplayID(), (CFDictionaryRef)@{
+		(NSString *)kCGDisplayShowDuplicateLowResolutionModes: @YES
+	});
+	CGDisplayModeRef pickedMode = NULL;
+	for (id object in allModes) {
+		CGDisplayModeRef mode = (CGDisplayModeRef)object;
+
+		if (CGDisplayModeGetWidth(mode) == 1500 &&
+		    CGDisplayModeGetHeight(mode) == 1000 &&
+		    CGDisplayModeGetRefreshRate(mode) == 60) {
+			pickedMode = mode;
+			break;
+		}
+	}
+
+	if (pickedMode == NULL) {
+		NSLog(@"Mode not found!");
+		return 1;
+	}
+
+	CGDisplayConfigRef config;
+	if (CGBeginDisplayConfiguration(&config) != kCGErrorSuccess) {
+		NSLog(@"CGBeginDisplayConfiguration failed!");
+		return 1;
+	}
+
+	if (CGConfigureDisplayWithDisplayMode(config, CGMainDisplayID(),
+	    pickedMode, NULL) != kCGErrorSuccess) {
+		NSLog(@"CGConfigureDisplayWithMode failed!");
+		return 1;
+	}
+
+	if (CGCompleteDisplayConfiguration(config, kCGConfigurePermanently) !=
+	    kCGErrorSuccess) {
+		NSLog(@"CGCompleteDisplayConfiguration failed!");
+		return 1;
+	}
+
+	return 0;
+}