Size does matter....at least with capacitors.

[52 Bill]

Something like this.
https://www.homedepot.com/p/3-in-x-1...3112/100343439

This one is way too big for what you want, but it explains what I meant.

There may be something that will work for you in your local plumbing store.

[Axtman]

Hmmmm. I will have to mosey over to my local hardware store and see what they got.

[bob p]

The plumbing supply companies also make rubber step-down adapters for to connect various sizes of non-pressurized (drainage) pipe. But your capacitor is at the lower range of sizes for those rubber plumbing adapters. An OD of 1.375" might be available in a fitting that connects a 1" or 1.25" ID pipe to a larger pipe. If that's the case you'd end up cutting the adapter at the small side to make a rubber ring. It seems that Dude's foam gasket method would be just about as good.

[Chuck H]

If you wanted to keep the stock appearance you could always cut open the old cap and stick the new, smaller one inside. This has been done quite a lot. Though I haven't troubled to link you shouldn't have trouble finding threads or tutorials. Then you could just use the hardware and mounting as stock.