Years ago at my friend's VW shop in NJ we had this discussion so we had a running clapped out VW pan fiberglass buggy , I cut a 3" x 5" three sided opening on the passenger side at the cable area bent it down which is no an easy task considering gauge of the tunnel metal, bent it back into place with the cut off wheel margins remaining on the sides... then took in to a construction site and ran it hard over hills turns etc. Back at the shop the opening margins were the same and that was w/o screwing or welding it back in place. IMHO no real issue doing an access panel as the tunnel aka Backbone is very strong.