With a POD after 1900 your challenge is to find a way for ugly straight line international borders as seen in Africa and in sparsely inhabited areas.
If you find this too difficult your secondary challenge is to have spheres of influence made in Siberia that follow similar principles.
Finally then if it's still too much of a challenge have these borders be temporary claims such as people allies in war or something and never reflect the actual situation.
Here is an example below.
Russian Civil War goes haywire and Turkestan seizes everything East of the Urals. Mongolia and recently independent Manchuria are allies with Turkestan against China(???) and so they quickly agree to borders so they focus their attention elsewhere. Japan grabs Kamchatka during the chaos as there are like 5 people there and no one contests it as well as Sakhalin.
If you find this too difficult your secondary challenge is to have spheres of influence made in Siberia that follow similar principles.
Finally then if it's still too much of a challenge have these borders be temporary claims such as people allies in war or something and never reflect the actual situation.
Here is an example below.
Russian Civil War goes haywire and Turkestan seizes everything East of the Urals. Mongolia and recently independent Manchuria are allies with Turkestan against China(???) and so they quickly agree to borders so they focus their attention elsewhere. Japan grabs Kamchatka during the chaos as there are like 5 people there and no one contests it as well as Sakhalin.