The pre WWII border between Poland and Germany was defined in the Treaty of Versailles, more specifically Part II, Article 27, point 7:
From the point defined above to a point to be fixed on the ground
about 2 kilometres east of Lorzendorf: the frontier as it will be
fixed in accordance with Article 88 of the present Treaty; thence in a
northerly direction to the point where the administrative boundary of
Posnania crosses the river Bartsch: a line to be fixed on the ground
leaving the following places in Poland: Skorischau, Reichthal,
Trembatschau, Kunzendorf, Schleise, Gross Koscl, Schreibersdorf,
Rippin, Furstlich-Niefken, Pawelau, Tscheschen, Konradau,
Johallnisdorf, Modzenowe, Bogdaj, and in Germany: Lorzendorf,
Kaulwitz, Glausche, Dalbersdorf, Reesewitz, Stradam, Gross Wartenberg,
Kraschen, Neu Mittelwalde, Domaslawitz, Wedelsdorf, Tscheschen Hammer;
thence the administrative boundary of Posnania northwestwards to the
point where it cuts the Rawitsch-Herrnstadt railway; thence to the
point where the administrative boundary of Posnania cuts the
Reisen-Tschirnau road: a line to be fixed on the ground passing west
of Triebusch and Gabel and east of Saborwitz; thence the
administrative boundary of Posnania to its junction with the eastern
administrative boundary of the Kreis of Fraustadt; thence in a
north-westerly direction to a point to be chosen on the road between
the villages of Unruhstadt and Kopnitz: a line to be fixed on the
ground passing west of Geyersdorf, Brenno, Fehlen, Altkloster, Klebel,
and east of Ulbersdorf, Buchwald, Ilgen,Weine, Lupitze, Schwenten:
thence in a northerly direction to the northernmost point of Lake
Chlop: a line to be fixed on the ground following the median line of
the lakes; the town and the station of Bentschen however (including
the junction of the lines Schwiebus-Bentschen and Zullichau-Bentschen)
remaining in Polish territory; thence in a north-easterly direction to
the point of junction of the boundaries of the Kreise of Schwerin,
Birnbaum, and Meseritz: a line to be fixed on the ground passing east
of Betsche; thence in a northerly direction the boundary separating
the Kreise of Schwerin and Birnbaum, then in an easterly direction the
northern boundary of Posnania to the point where it cuts the river
Netze; thence upstream to its confluence with the Kaddow: the course
of the Netze; thence upstream to a point to be chosen about 6
kilometres southeast of Schneidemuhl: the course of the Kuddow; thence
north-eastwards to the most southern point of the reentant of the
northern boundary of Posnania about 5 kilometres west of Stahren: a
line to be fixed on the ground leaving the SchneidemuhlKonitz railway
in this area entirely in German territory; thence the boundary of
Posnania north-eastwards to the point of the salient it makes about 15
kilometres east of Flatow; thence north-eastwards to the point where
the river Kamionka meets the southern boundary of the Kreis of Konitz
about 3 kilometres north-east of Grunau: a line to be fixed on the
ground leaving the following places to Poland: Jasdrowo, Gr. Lutau,
Kl. Lutau, Wittkau, and to Germany: Gr. Butzig, Cziskowo, Battrow,
Bock, Grunau; thence in a northerly direction the boundary between the
Kreise of Konitz and Schlochau to the point where this boundary cuts
the river Brahe; thence to a point on the boundary of Pomerania 15
kilometres east of Rummelsburg: a line to be fixed on the ground
leaving the following places in Poland: Konarzin, Kelpin, Adl.
Briesen, and in Germany: Sampohl, Neuguth, Steinfort, Gr . Peterkau;
then the boundary of Pomerania in an easterly direction to its
junction with the boundary between the Kreise of Konitz and Schlochau;
thence northwards the boundary between Pomerania and West Prussia to
the point on the river Rheda about 3 kilometres northwest of Gohra
where that river is joined by a tributary from the north-west; thence
to a point to be selected in the bend of the Piasnitz river about 1
1/2 kilometres north-west of Warschkau: a line to be fixed on the
ground; thence this river downstream, then the median line of Lake
Zarnowitz, then the old boundary of West Prussia to the Baltic Sea.
Ref: http://avalon.law.yale.edu/imt/partii.asp#art27
As you see the border is only defined approximately with an accuracy about a km or so, but that should be good enough for most mapping exercises. Another good resource is Wikipedia, which also points out a minor change in 1921.