I should also make myself a redneck version of Brodie's gauge. But I'm not sure it's so necessary to align the front triangle apart from twist, and I've never had it twist, at least not measurably by me.
Basically I get the BB shell as square as I can onto the bottom of the ST using a combination square with the ruler taken out. Then I get the dropouts and BB shell parallel using the laser and angle iron bolted to the table. Then I jig it up for the front triangle with everything laser-vertical. Then I make sure the ST is centered between a bit of string wrapped around the HT and the dropouts. After welding the front triangle the string can be recentered if necessary a small amount just by moving the chainstays both left or right together.
Compared to bending the front triangle this will leave the dropouts slightly misaligned and the BB shell slightly rotated around the vertical from where it should be. But the BB shell is only 68mm long compared to the 1m or so over which we're measuring the alignment with the string. So it's only a tiny change. All of the bikes I've made have actually ended up with squarer BBs than any of the commercially-made bikes I've owned.
A higher priority tool for me would be a precision "wheel" consisting of an axle and a perfectly square and centred rod to sweep up and down between the stays perhaps with an adjustable thingy that you could set to just brush against them on both sides. Currently I rely on a true and properly dished wheel which is usually a brand new one I've just made for the bike. But there's lots of agonizing and flipping of the wheel.
I think the best way to make precision tools without machining capability is to make things that can be strategically twisted and bent a bit so you can make them, check them with a laser, and tweak them as required. I also have some things a bit like yours with bolts that can be used to push things in and out a bit.