Some Bongo owners have had mattress blocks made for their van bed arrangements. Here are some places mentioned by Bongo owners as being worth considering.
D&G Auto Trimming, Newcastle – Washington (facebook page)
Lidl – When they have car seat covers in stock, they apparently are a great fit for Bongo seats
Wessex Car Trimming Ltd, Southampton – used by a Bongo owner on a few occasions and highly recommended. You can even supply your own foam to be trimmed.
JDS Metal Tech, Sheffield (Rock and Roll beds)
Rusty Lee’s Hand Made Rock N’ Roll Quality Beds, Hereford (Rock and Roll beds)