You might want to consider using NDebugOverlay stuff instead of beams. NDebugOverlay includes...
Code:
void Box(const Vector &origin, const Vector &mins, const Vector &maxs, int r, int g, int b, int a, float flDuration);
void BoxDirection(const Vector &origin, const Vector &mins, const Vector &maxs, const Vector &forward, int r, int g, int b, int a, float flDuration);
void BoxAngles(const Vector &origin, const Vector &mins, const Vector &maxs, const QAngle &angles, int r, int g, int b, int a, float flDuration);
void SweptBox(const Vector& start, const Vector& end, const Vector& mins, const Vector& maxs, const QAngle & angles, int r, int g, int b, int a, float flDuration);
void EntityBounds( const CBaseEntity *pEntity, int r, int g, int b, int a, float flDuration );
void Line( const Vector &origin, const Vector &target, int r, int g, int b, bool noDepthTest, float flDuration );
void Triangle( const Vector &p1, const Vector &p2, const Vector &p3, int r, int g, int b, int a, bool noDepthTest, float duration );
void EntityText( int entityID, int text_offset, const char *text, float flDuration, int r = 255, int g = 255, int b = 255, int a = 255);
void Grid( const Vector &vPosition );
void Text( const Vector &origin, const char *text, bool bViewCheck, float flDuration );
void ScreenText( float fXpos, float fYpos, const char *text, int r, int g, int b, int a, float flDuration);
void Cross3D(const Vector &position, const Vector &mins, const Vector &maxs, int r, int g, int b, bool noDepthTest, float flDuration );
void Cross3D(const Vector &position, float size, int r, int g, int b, bool noDepthTest, float flDuration );
void Cross3DOriented( const Vector &position, const QAngle &angles, float size, int r, int g, int b, bool noDepthTest, float flDuration );
void Cross3DOriented( const matrix3x4_t &m, float size, int c, bool noDepthTest, float flDuration );
void DrawOverlayLines(void);
void DrawTickMarkedLine(const Vector &startPos, const Vector &endPos, float tickDist, int tickTextDist, int r, int g, int b, bool noDepthTest, float flDuration );
void DrawGroundCrossHairOverlay();
void HorzArrow( const Vector &startPos, const Vector &endPos, float width, int r, int g, int b, int a, bool noDepthTest, float flDuration);
void YawArrow( const Vector &startPos, float yaw, float length, float width, int r, int g, int b, int a, bool noDepthTest, float flDuration);
void VertArrow( const Vector &startPos, const Vector &endPos, float width, int r, int g, int b, int a, bool noDepthTest, float flDuration);
I'm not sure if/how NDebugOverlay would work from a plugin, but it seems like a much better system for drawing "debug" lines/boxes/text/etc in the game.