There are a few decent sized side-trays with a steadying leg which also have a swivelling attachment to make them level - have a look at the newer Preston products, which are the same fit. Failing that, Korum and Wychwood (and others, presumably) have holdalls and backpacks with a moulded table area on one side. I've been using an el-cheapo bivvy table that was a tenner - it's as good as anything else.