Use the Tcl 8.0.5 installer.
Add a variable %_TCL_% that makes it easier to switch to a different version.
diff --git a/PCbuild/python15.wse b/PCbuild/python15.wse
index 9ee79fe..f76e7c0 100644
--- a/PCbuild/python15.wse
+++ b/PCbuild/python15.wse
@@ -30,6 +30,9 @@
   Variable Description4=HTML documentation tree

   Variable Default4=D:\src\Python-1.5\html

   Variable Flags4=00001000

+  Variable Name5=_TCL_

+  Variable Default5=tcl805.exe

+  Variable Flags5=00001000

 end

 remarked item: Open/Close INSTALL.LOG

   Flags=00000001

@@ -1250,14 +1253,14 @@
   Flags=00100001

 end

 item: Install File

-  Source=D:\downloads\tcl804.exe

-  Destination=%TEMP%\tcl804.exe

+  Source=D:\downloads\%_TCL_%

+  Destination=%TEMP%\%_TCL_%

   Description=Tcl/Tk installer

   Flags=0000000000000010

 end

 item: Execute Program

-  Pathname=%TEMP%\tcl804.exe

-  Command Line=tcl804.exe

+  Pathname=%TEMP%\%_TCL_%

+  Command Line=%_TCL_%

   Default Directory=%TEMP%

   Flags=00000011

 end