No link to a video.  However, I've made them out of hardwood (or maybe a scrap of teak), just to avoid any concerns about delamination.   First I dry fitted the parts, and drilled the hole for the through hull itself perhaps a 1/4" oversize, so that there was room to all multiple layers of epoxy.   Same with the bolts for the seacock.  I then coated them with maybe 6 coats of epoxy, making sure to brush it well into the end grain and into the holes.   You'll no doubt have to drill out the bolt holes again.  
If you do use plywood, pay special attention to sealing the edges to prevent delamination.

Mainesail has a couple of good illustrated pages on this process (he recommends making your own fiberglass backing plates: http://www.pbase.com/mainecruising/replacing_thruhulls
, http://www.pbase.com/mainecruising/backing_blocks and he's no doubt correct that in the amount of time it takes to pay on 6 coats of epoxy on your wood block you could make a backing plate from fiberglass from scratch.

You beat me to the answer.  My boat's PO had installed a wood backing plate (not through bolted) to install after-market air conditioning.  On my first seacock inspection, the block came off in my hand!  I think the combination of drying wood and ill-prepared bedding loosened it from the hull.  I read all of Mainesail's instructions (great!), but decided against making my own fiberglass block or using wood.  I bought a Groco block on eBay (Groco Seacock Backing Block 3/4" - 1" BB-1), and it worked like a charm.  I think if you're installing dozen blocks, it makes economic sense to make your own, but if just doing one at a time, the $37 for one that fits perfectly is cheap insurance.

I don't know your particular situation, but for documentation for others that may read this thread later ... The Groco backing plate has two sets of mounting holes with different spacing.  When rotated and reversed, the second set fits the larger seacock valve.  The center hole, and the throughhull hole in your hull, should be slightly (1/8 - 1/4 " per side) larger than the throughull fitting to allow packing with a more flexible underwater sealant (3M 4000 or 4200, for example). This allows some give and prevents leaks. Again, not saying that in your case it is not wrong, just don't want to leave the impression that everything fits snuggly.

As far as instructions, I used Mainesail's excellent write up for non-through-bolted through-hulls (see http://www.pbase.com/mainecruising/boat_projects  I used West Systems Six 10 as permanent adhesive and it worked extremely well. It's really just thickened 2-part epoxy, but applies using a caulking gun so no mixing required.  I feel that it's as strong as the hull.  The 5200 is a good solution as well due to it's incredible high adhesion.  I used the seacock valve to hold the backing plate in place while the stuff set. I took care to wax both the seacock and surrounding hull with carnuba wax so I could remove it later.  These seacocks eventually fail, so you want to be able to replace it later without chipping out and replacing the entire block.  There is fiberglass molding wax available on-line, but paste carnuba car or boat wax will work as well. Remove the wax using acetone (lots) prior to applying the adhesive as nothing sticks to it.  Good luck.