I like the old Regula Variant 740 flashes (from the 90s or earlier, using 4×AA batteries and working fine with 1.2V NiMH rechargeables). The 740-2MFD has a trigger voltage below 12V. The 740CTX I have measured with something like 180V or so, so unless you do a proper separation circuit, it's probably to be avoided.
It has the typical "cobra head" form factor and a guide number of 40m at ISO100. The 740-2MFD allows 5 manual settings from P/1 to P/16. Most importantly, however, for the sake of drowning out the sun is that it is specified to be 1/1000s at P/1. Many more modern flashes are 1/200s at full power, making it much harder to drown out the sun because of the longer exposure needed.
Making best use of such short flashes assumes that your camera offers electronic shutter and consequently arbitrary flash sync speed.
Downsides are that its specified for f=35mm equivalent FOV. There is an optional wide angle attachment available with light scattering panes for f=28mm (-1EV) and f=24mm (-2EV). However, acquiring extra bits like that means extra hunting for old equipment: most offers don't include attachments. But you'll find yourself in the same predicament regarding almost any old flash without a zoom head. Of course a more efficient remedy for a fixed contraption like the one you are planning is just to move the flashes backwards until they cover the area you want.
Metz 40MZ-1 and 40MZ-3 are nice in that you have 25 manual settings (P/1 to P/256 in EV/3 steps). They zoom down to 24mm. While their guide number of 40 is specified for f=50mm, at f=24mm (the lowest of their zoom ranges) I think they are even stronger than the Regula with 24mm attachment. However, they are specified with 1/200s at P/1. 40MZ-2 will also zoom to f=24mm. Unlike its cousins 40MZ-1 and 40MZ-3, it only zooms up to f=70mm instead of f=100mm, and it does not have a distributor panel built-in that you can manually engage for f=20mm.
I usually use an additional reflector (probably taking 3EV all in all) for closeup photography with my Regula, and 1/2000s of exposure at F16 and ISO160. That does still drown out natural light sources pretty well. Most of the time, the flash operates at P/8.
However, that is at a shorter distance than what you are planning to use (maybe 20cm from the reflector?).