My mother-in-law wants me to try and attach this metal cross to the wood cross below so she can hang it on the wall. Trying to figure out the best way to do that. Any and all suggestions are welcome.
Then I suggest 6-32 screws. This bottom tap from Amazon is only $5.99. You don't need a full set (unless you just want one, and can find a set of bottom taps in a set. A t-handle tap wrench can be had for about the same price at about any Ace Hardware. You'd also need the correct size drill bit for 6-32 machine screws, which you can research with google. If you are looking for a free solution, it would tough for anyone who doesn't know what you do and don't have.
I'm sure there are any number of other ways to approach this, but the suggestion I made was just what I'd do for a secure attachment.
I think I would drill counter sunk holes through the cross, back to front, drill and tap matching holes in the back of the metal, and use machine screws to attach the two pieces. I can't see how big the cross is, but I'm going to guess 8-24 screws would be sufficient. Just use an 8-24 bottom tap to thread it so that you don't have to drill too deep a blind hole in the metal part.