i was thinking of putting some backing on the reel, something soft like perhaps a layer of duct tape

I am constanly having to adjust my reel seat on the st. croix mojo bass..Im sure im not the 1st, i can tighten it but it will not stay like that for long.

any old tricks?

It's a common problem with the lower-end St. Croix rods (Mojo, Premier, Triumph) Loosen the locking nut all the way and give the threads a couple wraps with teflon thread tape. It'll take up the slack between the nut and threads but won't leave any sticky crap if/when you decide to remove it.

It's not just the lower end rods. I sold my LTB spinning rod because of the same problem. I wish I hadn't it was a great rod, I just got tired of tightening the seat.

I have a similar problem on some of my rods where the reel seat won't tighten up because the feet of the reel are too small. I take some cardboard from something like a cereal box and cut it to fit the length of the feet and then put the reel on the rod. Fixes the issue every time.

I have over a dozen mojos and premiers and have never had that problem. But surfcasters sometimes due because of the torque they make with those crazy casting techniques. They put a few wraps of clear packing tape on the reels feet. It builds up to make a better fit with the inside of the hood. Not St. Croixs fault but you'd think Fuji, the premier rod part maker would have better quality control.