Prohands Gripmaster Hand Exerciser, Finger Exerciser (Hand Grip Strengthener), Spring-Loaded, Finger-Piston System, Isolate & Exercise Each Finger

Honestly would prefer a grip master thingamabob over this- the ones that exercise the individual fingers- like this guy https://a.co/d/08T2bMPT - I find that coordination should be trained with the guitar, but endurance and getting to a point where you’re able to hold chords without straining would better come from something like this.