An adjustable cable is better. That was my goal, but I never made it out to the boat rigging store to make a stainless cable with turnbuckle, just like Dahon's Deltech, and with ultra-high-strength flexible loops around the head tube and bottom bracket, to connect to (Dahon has small welded fittings there, and I actually think that may be a weak link in the system on their aluminum frames).

Well, I happened to have a piece of kevlar line laying around, just long enough for the loop you see. How did I get it tight? I tied it with the bike slightly folded, gently started to unfold it to check tension, nope, not quite tight enough, shortened it, just enough so that I had just a bit of difficulty fully unfolding the frame, so laid it flat and gently used my foot to snap it to fully unfolded, tension was perfect, and has not loosened the tiniest bit, nor stretch, in 3+ years now. (You need to be careful with the above, because once that 8 bend is really tight, it'll be a bear to get undone. Needs to get the length right before that knot goes tight.) But an adjustable setup would be easier. I could also do same loop but add some small loops to do a single or double or triple trucker's hitch; These days, many round-the-world sailboats are replacing stainless steel cable standing rigging with swaged fittings (which can't be repaired as sea, unless you brought replacements with you), for high strength line (usually one of the brands of UHMWPE these days) with multiple "blocks" (pulleys) in 2/3/4:1 or more for enough leverage to tension up the sailing rigs, no turnbuckles, simpler, and fatigues less than stainless cable and end fittings. Lighter (I think 1/7 the weight of steel for same strength), very durable, nearly zero stretch, no creep over time, no corrosion, UV resistant, and easy to replace at sea, just standardize and have a spool of that line on board, cut to length. UHMWPE is also UV resistant, whereas Kevlar is not, it's actually off-white and the line you see on mine is actually painted black on the outside by the maker for UV protection. But Kevlar is almost completely supplanted in the market by UHMWPE, which also comes in multiple groovy colors.

With the ovalized holes as you mentioned... deltech cable will force the hinge closed tightly, and loading the pin in shear instead of bending couple, but if the holes are large, I don't know if, with the hinge closed tightly, it will be tight on the pin top and bottom, or not. I did my deltech soon enough that the hinge is drum-tight when unfolded. Noticeable difference; Gone is clunk-clunk slack with front brake on and rocking bike back and forth, and frame is a lot stiffer at the hinge when climbing standing, which puts the hinge in torsion.