I've just taken the lid of my G68 again and measured the distance between the bottom of the struts and the front of the case more accurately. Taking into account the protruding connectors which connect the front two boards (these are at a height of approx 85 mm above the base), my original estimate was bit generous and it's more like 117mm.

At their base, the supporting stuts are 38mm wide so thers's scope for cutting or replacing these. I wouldn't want to go within the 20mm of the circuit board so there's approx 135mm of space to play with.

Main system - Sources (Kodi-HTPC, Vero 4k, FireTV, Sat) into Trinnov Altitude 32 (8-16) (+Roon), Trinnov Amplitude 8 amp & Vivid Audio (Giya G3, V1.5) and REL Stentor III speakers. Others: F80 + Raspberry Pi and other Pi systems.