If you specifically need version 8 for legacy projects, check reputable software repositories like OldApps.com or Internet Archive , though these are typically the standard installers rather than "portable" versions. System Requirements for SketchUp 8
Because there is no official portable release, files found on third-party sites are often community-made modifications. These come with several risks: Security Vulnerabilities Google Sketchup 8 Portable Free Download
Professional-grade additive and subtractive modeling (union, intersect, subtract) available in the Pro version. If you specifically need version 8 for legacy