You've gotten all the technical answers ... but keep it simple ....increase the weight of your gun ( yes you can tape a spent shell full of shot into the stock of your gun ) ....but don't let it move around. Get it set / so the balance point of the gun remains the same. Ideally, I like to add about 8 oz under or inside the forend / and 8 oz in the stock ... and adding 1 lb will reduce the recoil about 20%.
Shoot shells with 1 oz or 7/8 oz of shot ....and keep the velocity down to around 1200 fps or even drop it to 1150 fps ... If you reload - that's easy to do.
Expensive ..?? is relative ..../ personally, I've added a GraCoil system to my primary Trap gun - and it helps a lot. Mercury reducers work pretty well ...but so do the homemade solutions.