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
JDS Metal Tech, Sheffield (Rock and Roll beds)
Rusty Lee’s Hand Made Rock N’ Roll Quality Beds, Hereford (Rock and Roll beds)