I wasn't happy with the fit of my GX-56BB-1 King. Most of the time I'm between holes on the strap. On the larger position the watch flops around too much. On the smaller position my wrist is squeezed uncomfortably top to bottom. Since the strap holes are positioned a bit farther apart than on a typical G-Shock strap, this size difference is exaggerated.
I noticed that JaysAndKays is now selling Deep Lug Large G-Shock resin adapters for the GD-350, GD-X6900 and GX/GXW-56 (King), so I decided to give them a try. They arrived today.
These adapters fit with no problem on the GX-56, but they don't fit tightly to the watch case as do the 5600 Convertible adapters on the GW-9110 Gulfman. The adapters can rotate a bit (articulate) around the screw bars, which means they can likely accommodate a somewhat larger or smaller wrist.
On my wrist, with the provided 24mm J&K 3-ring Zulu strap, the fit is much better than with the stock King resin strap. This Zulu strap fits very snugly in the adapters, much more so than a 24mm Maratac Zulu in the stock Casio adapters or a 22mm J&K Zulu in the J&K 5600 Convertible adapters. The watch head will not slide out of position due to the snug fit on the strap, but it makes changing straps a little more time consuming.
As with other resin strap adapters, the excess strap tail can be pushed into the upper adapter, once the watch head is positioned properly on the strap.
I'm happy with the result. I'll probably wear my GX-56 more often now. I have a 24mm Vario single-pass seatbelt NATO strap on order. Once it arrives from Singapore, I'll try it on the GX-56.