Perhaps I am missing something here & I know this does not help right now, but........
Why don't these medium size van manufacturers (VW Transporter, Ford Transit etc.) just redesign the front seats so that they can sit higher in the cab with a thinner base?
Use different seat materials & perhaps at greater cost to manufacture.
This would then allow 8X4 sheets to slide under the front seats.
A steel retaining bar would obviously be required to stop the sheets sliding forward when the vehicle reduces speed.
The benefit of this in, say, a short wheel base VW transporter, would be that the entire vehicle could be 670mm shorter which, if put into production as an additional model, would open up an entire new market segment.
I refer to the small builder looking for the smallest vehicle that can transport half a dozen 8X4 sheets.
It seems to me that the excellent quality entire VW Caddy range could then be dropped from production as there would be a VW transporter body to satisfy every need including a disability vehicle.