In my prior post I metioned getting a piece of 2x6 and chopping it into 2 four-foot parts.
I set the first up on a routing table with a sacrificial spacer board between the 2x6 and the router table. Clamps held the board to the table and smaller ones held the saw guide straightedge in place.
I made a test pass with the circular saw a couple inches over the board to make sure my cord wouldn't get bound up or anything. Then, with the saw base set at a 45degree angle (according to the guide --not the most precise measure...) I started cutting, being careful to stay up against the guide. The first few inches were'nt all that straight, as expected, but then I had the whole saw against the guide and aligned straight. It went fine for a few more inches until I hit the first clamp. When I did the test pass I was too high and missed it but now on the real pass I was stuck. I pulled back an inch and let the saw spin down. Removed the clamp (it was tight enough with just the other one) and continued the pass. Swapped clamps when I fetched up against the second clamp. Then all was fine until for some unexplained reason I couldn't push the saw through the last four inches. I'm still not sure what I was hitting up against but I called it quits.
Using a big miter/chop saw I cut off the first few inches and last few inches, then from the remaining straight stock I cut it so the long side was around 6 inches bigger than the side. Shown is the shorter waste piece and the two pieces I will use.
I then aligned the two good pieces as well as possible, and checked them with a 90 degree angle (2x45 should add up to 90). It was clear that the angle wasn't perfectly set, but as long as top and bottom were equal (from being part of the same stock) it should be fine. I clamped them together and put some 1/2" holes in and fed dowels through, then glued the dowels into the lower part.
Here are some pics in theoretical use. I just used some elastic I had on hand; I'll use a thicker piece after I pick some up. And that blue BT-60 is from an old blue ninja kit that I bashed various parts from. And the fin is a reject from another project, and I didn't put shims in yet (just open space in there...)
It actually looks pretty good, but unless I trim it a bit it won't work for 4 fin BT60-or-smaller projects due to overhang of the angle bit.
I will probably make another one out of thinner stock (1") which would be just as easy. Very good learning project, and this one would be fine for BT 70/80 project I'm considering.
Oh, I didn't put wingnut hold-down screws in this yet... I may or may not do it on this one but probably will on my next version.