You will be tough pressed to find much info on plans or measurements for a bracket build, not many do them and the ones that do it for a living its not in there interest to teach others how to build their own. Member BigShrimpin up here will be willing to help as I will as well. There are others that have built their own but very few.
When I build mine I match the transom angle of 13 degress to the back of the bracket outboard mount and run them parallel. The swim platform top is on the same plane as the boat deck or slightly elevated at the rear aft. The bottom I have done it two different ways, one matching the center panels at the same degree and one matching the bottom vee panels at 20 degrees. It will look better and gain better flotation if the bracket matches the panels and the boat vee. It mainly depends on how wide you make it to which panels you match. If you make the floatation tub the same width as the outer second panels I would match them. If you make it more narrow I would match the bottom panels like the hermco brackets do. I also built an 20 degree rake on my tub upwards to the rear of the outboard mount side so that as the water rises it will not act as a huge trim tab. This works out to be ruffly a 2" grade up to the rear of the bracket with a 2 foot set back. Doing this will allow you to mount it lower. Most metal brackets have this built in but the fiberglass ones I have seen look to be straight back with out a grade upwards. That may cause you to mount it higher to clear the water passing by it on plane. You loose some volume with the grade but then you gain it back by lowering the mounting height so it will yield more floatation volume back. The volume value between the two styles may not yield much more lift but I like the better looks of the tapered tub and feel like it has less water drag. Setback need at least 24" with most being around 30" or more to allow the motor to go to full tilt. Biggest thing when setting it up to look at is (1) motor height (2) swim platform height & height mounted to the boat (3) How high from the bottom of the hull you will mount it. (4) setback for motor tilt. I start with those 4 factors and then figure the dimensions in between to get the over all height, tub height, tub depth, motor mount ear height, and the angles of the transom and the angle of water rise (about 1.25" for 12" of setback) to determine the other angles that are not matching the hull bottom. Width is a preference but you can gain floatation value to a point then after that it max out then more just adds weight and no lift value.
On post #55 of the thread below there is a sheet on the last picture of some basic dimensions, but these are for a 25' hull. I have the dimensions for a 20' hull bracket that was on the 20' you saw the other week. I can send you those measurements via PM, just sent them to another member a few weeks ago. The 20 bracket I have will mount up around 2.5-3" from hull bottom for maximum floation and put the swim platform up about 2" above the water line while still giving you the correct motor mount height to get the outboard av fin in clean water while running.
http://www.classicseacraft.com/commu...t=26635&page=2
Like said I will try to sketch a ruff drawing of mine on my 20' hull and give you the basic dimensions and you can modify it to you likings. Seems like most people with brackets raise their motor high as possible and run out of adjustment before they get the outboard high enough. You will need about 4-4.5" of distance from the keel to the av plate depending on setback. Give your self enough tab to mount the outboard high enough to achieve this but at the same time watch out cause if you keep the swim platform close to the water height you will find out that your lower motor mounting bolt holes to be so high they end up needing to be drilled about where the foam or wood in the swim platform is. Mine are all the way as tight up as possible to the bottom of the platform and just barely work out getting the motor height right around the last holes of adjustment. I think mine are one hole down form the highest setting. To get any higher the platform would need to be raised and then you get it so far above the water line it is not as useful getting in the boat out of the water. It don't look as good high either. Also seal the lower bolt holes well or you will get water leaking into the tub from your lower mounting holes.