you could jsut screw standoffs into the wood but most mobos are earthed through the metal around the holes intot he case so you just need to have all these points linked and earthed. Might be easier to get an old back panel off a case (the bit the mobo bolts to) and screw this down 1st. If you hack up a case you can get the pci card supporting bit too.
EDIT: jsut gonna make myself a bit clearer:
If you have an old case with the side the mobo and the back face were all the cards bolt to and the back conencters come through just cut this (carefully as the thins steel is sharp) and bolt this down. This way you get the nice earthing (make sure the case section is then earthed to a common point along with other parts of your system) and you get good support for any PCI cards - if you have any. Loose PCI cards will cause you a lot of trouble with hanging and crashing. YOu might also want to brace the L shape as if it flexes even slightly then the cards will twist out. Thats why cheap cases are crap and cause loads of trouble.