My spoil board choice solved itself today. I had to take the car in for a firmware update (I kid you not) and whilst I as waiting I noticed a small plastic engineering shop over the road. Knocked on their door and the owner was absolutely flat out making perspex screens for covid protection in shops and offices, he said he was 10x busier than he'd ever been before although there was now a world shortage of perspex, well done him!

Anyway I asked about foamed PVC and he went into his warehouse and pulled out a 19mm offcut 1300x700 which he sold me for £30, perfect!
Click image for larger version.  Name: IMG_20200605_154001 (Large).jpg  Views: 1123  Size: 224.2 KB  ID: 28303