To anyone considering fitting a simple brake like a figure 8, whether or not you make your own (and I've made a slew – it's not difficult): the one simple act which lifts the braking controllability and performance more than any other is to rig it as recommended for the ServoBrake.
(which itself is a more sophisticated and very rugged alternative to the sort of descender you posted - perhaps the key attribute is that it causes much less line wear - photos below show the generous rope
groove taking the fixed line through the entire two-turns odd)
Note that such brakes rely on being paired with a powerful kicking strap / centreline vang.
I attach a couple of clips showing the Servobrake setup principle.
The key is that the black line is fixed at both ends, abeam of the mast
. The blue control line runs around turning blocks alongside those end fixings, and back to a decent clam cleat (small boat) or clutch
(big boat) at the cockpit
. The cleat doesn't see anything like gybing loads, but it's still appreciable.
I became a believer in this setup after being forced (in order to avoid an island I'd misplaced) to carry out a gybe in the notorious Cook Strait in a heavy squall with 35 knots across the deck
under full main on a generously canvassed modern 40 footer. Alone. At night.
Somewhat to my relief, it was entirely drama (and shock) free.
I rate this brake more highly than either Walder (and clones) or Wichard, mainly because of the better control from the way it's rigged. (In engineer-speak, it gives better control over the lesser of the T1/T2 pair)
If there's any possibility of dipping the boom at speed, I recommend fitting a 'fuse' connecting each control turning block to the chainplate: something which fails in order to spare the boom breaking, but only lets the boom come inboard uncontrolled a short distance before the brakes come back on and decelerate the further inswing. Such as a weaker lashing making a short loop, inside a much stronger lashing of nylon cord (for shock absorption) which is longer by a predetermined amount to create the behaviour I describe.