Extra wide and well padded the Tanker is a Rob Allen ultimate speargun transit bag. Designed to withstand even the roughest handling the Tanker also features an internal spear pouch to carry spears complete with rigging lines. The tanker can also accommodate fins and other freedive gear. This will hold 5 spearguns, possibly more.
The next step up from here is a hard case which has a weight penalty some people like to avoid in case they go abroad.