Here is some working code to make a 7zip of a bin folder

7zip.targets

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup>
    <TargetSevenZip>$(TargetDir)\$(TargetName).7z</TargetSevenZip>
    <!-- See http://msdn.microsoft.com/en-us/magazine/dd419659.aspx#id0090093 -->
    <CleanDependsOn>
      CleanArt;
      CleanSevenZip
      $(CleanDependsOn);
    </CleanDependsOn>
    <SevenZipDependsOn>Build;</SevenZipDependsOn>
  </PropertyGroup>
  <Target Name="CleanSevenZip">
    <Delete Files="$(TargetSevenZip)"/>
  </Target>
  <Target Name="7zip" DependsOnTargets="$(SevenZipDependsOn)">
    <PropertyGroup>
      <SevenZipExePath>$([MSBuild]::GetRegistryValueFromView('HKEY_LOCAL_MACHINE\SOFTWARE\7-Zip', 'Path', null, RegistryView.Registry64))7z.exe</SevenZipExePath>
    </PropertyGroup>
    <Message Importance="Low" Text="7Zip executable: $(SevenZipExePath)"/>
    <Error Condition="'$(SevenZipExePath)' == '' Or !Exists($(SevenZipExePath))" Text="7zip does not appear to be installed." />
    <Exec Command="&quot;$(SevenZipExePath)&quot; a -t7z -mx9 -x &quot;poshrunner.vshost.exe&quot; &quot;$(TargetSevenZip)&quot; &quot;$(TargetDir)*&quot;"/>
  </Target>
</Project>