If a company vouchers an inventory invoice in march but doesn't physically receive those materials until April what does the adjusting entry look like?

I don't consider this an accrual, because simply receiving the invoice hasn't caused an economic event. If you aren't receiving the materials until April, then just receiving the invoice doesn't have any meaning.

There's two things you have to consider. Will you own the materials even though you have not physically received them yet? Or, must you pay for the materials from the invoice before you receive them? That last would be a deferral, not an accrual.

An accrual is when a revenue has been earned or an expense incurred, but not yet paid. i.e. the "event" happens prior to the cash. All that I see happening here is receiving a piece of paper. If you insist on recording the invoice into your payables, you can't record the materials you have not received yet, unless it's a situation where you already own them anyway.