The width of the base blank is determined by combination of: the distance of the edge of the saw's base to the blade at its widest side + the distance of the saw's base to the blade at its narrowest side + the width of the aluminum U-channel + 1/2". The length is determined by the longest piece you plan to cut + 2". In my case, I use my table saw to rip 4' x 8' sheets of plywood down its length and my circular saw to crosscut along it's width, thus 48" + 2" = 50" long.
The length of the reference fence is the same as the base. To attach the fence to the base, I first drew a reference line on the base to line up the fence. The distance was the distance of the edge of the saw's base to the blade at its widest side + 1/4". In my case 3-21/16" + 1/4". Then I clamped the fence (upside down) to the base along the line and drilled 9 holes evenly spaced sized for #10 machine screws through both pieces. It's best to use a drill press if you can. Adjust it to a slower speed, and use lubrication. Take your time on this. Also, using high speed steel drills bits or titanium coated bits is recommended. I countersunk the base piece from the bottom so the head of the screw is just below the surface of the base. Then with lock washers and nuts, assembled the fence and base together. Make sure the length of the machine screws do not protrude past the height of the reference fence.
For the next part, make sure you have a carbide-tipped blade (it will cut aluminum easily and not dull it), hearing and eye protection, and gloves. Secure the cutting jig to a good surface with clearance for the blade. Start with the wide side. Place the front your circular saw firmly and squarely on the base and against the fence, but with the blade clear of the base. Fire up the saw and let it come up to speed. SLOWLY but STEADILY cut the overhang of the base (should be about 1/4") off the full length of the guide. When done, you should have a good reference edge (with the added benefit of being a zero clearance). Repeat for the narrow side. That way you can use either sde of your circular saw. It's always preferable to use the wide side, as it has more support, but it's nice to have the narrow side if you need it. After the saw guide jig was cut, I used a very fine sanding paper to ease off the sharp edges of the jig wherever I could. Trust me, those edges can cut deep if you don't.
I'm including a rough, not-to-scale drawing of what I explained above. I hope it helps.
prod miked into a D-I-V-O-R-C-E for that circular saw.
