You may struggle with that, the area doesn't really lend itself to free Cali camping with roof up.
Merchant Taylors is situated in a gated community of expensive houses and private security, complete with manned gatehouses. The school itself is an obvious option if that's open to you.
There is a very good pub not far away which does great food. I've not seen anyone camp in their car park but they may be worth asking.
The Rose and Crown
Harefield Rd, Rickmansworth WD3 1PP
01923 773826
Another to try is the Old Orchard at Harefield who have a large and quiet car park.
Otherwise I'd suggest looking at proper campsites, not that we have many hereabouts. Whilst we are next to the greenbelt with lots of rural car parks etc dotted about I don't think you'd get away with your roof up.
I'd offer our drive but it's full already and is so steep you'd not sleep....