I use a “Zen Strength Adjustable Hand and Finger Exerciser” that sells for about $15 at Walmart. It is a five-button squeeze exerciser. Each of the five is independently adjustable. Since most humans have one thumb and four fingers (me included), I adjusted four of them to the same tension, and the fifth one to about double that. The four are used as a typical squeeze hand exerciser. The fifth one is for my trigger finger. If I work at it, I can hold it tight and use just the one finger without dropping it. Works great!
Having read much of the above, though, the dry fire suggestion seems the best idea yet.