Mounting Plate

[blackburn mark]

if you can get hold of a longer 1/4 or 3/8 (or what ever your tripod head mount thread is) it wont matter if you use 8mm (ish) aluminum plate to keep the weight up

life would be a bit simpler if you had drawings or at least a photo of the plate from above
even a rough drawing not to scale but with dimentions would probably get you a quote

good luck

EDIT: iv just had another look at the plate you linked.... its 17.8 x 10.8 cm i cant see any need for you to fixate on any perticular size or hole pattern though.... you just need to work on what size holes and slots you want and decide where you want them, im assuming you will want a similar pattern so if you are tracking with a long lense you can keep the centre of weight between the wheels