They are never going to fund making bomb shelters, for a simple reason: Hamas Tunnels. Hamas has an incredibly extensive tunnel system, and the stuff you'd used to create a bomb shelter is what you'd use to create more tunnels. Hamas, if they wanted to, could have bomb shelters and protect civilians, but they don't want to protect civilians. To them, both Hamas fighters and regular Palestinians dying because of Israeli Airstrikes are desirable things to aim for. First, it makes them a Martyr, which is an instant trip to Paradise. Second, it is how Hamas wins the PR battle with the greater world.
Hamas, for the record, says this. They actively say that death is their strength, that they want their children to be martyrs, and that Israel's weakness is their unwillingness to die.
This is what some mean when they call Hamas a death cult, btw. It's when death of one's own side becomes a positive good to aim for.
So there are no bomb shelters, because no one wants them. Hamas will use any materials for tunnels instead, Israel knows this and thus doesn't allow such materials, and nobody asks because they know exactly what Hamas will use the materials for.
Even if a bomb shelter was supplied to Gaza in bombshelter form, for example if it was created in Gaza under supervision, shortly afterwards it will be tunneled into and used as another entrance to the Gaza Metro.