Fattening a Cat:
Why? Well cancer wastes them, heart disease wastes them and bringing them back from hyperthyroid states are three common reasons. There are more.
Tuna in OIL? Would be good. Warm vanilla icecream. Sweetened condensed milk. Scrambles eggs drowing in butter...Soft boiled eggs with yolk beat up with salt.....bacon crispy and bacon soft.....Dark meat chicken and the drippings. Rotisserie chicken light or dark meat and skin. Salmon raw, salmon oil/butter cooked, wagyu. Even some of the cheap canned foods are well accepted and VERY fatty - look at labels.
If You Want to Reduce Weight in a Cat Click Here (Illustrated)

Omega Paw Elite Self Cleaning Litter Box Large (Not electric - Not $499 either. LOL)
There's another one of these that sometimes opens when you roll it. Oops! But this one is upgraded with thicker material and better clips. You just roll it and it cleans itself into a scoop inside. PATENTED SELF CLEANING SIFTING GRATE Separates waste from clean litter. It uses clumping litter. It's super-highly reviewed on Amazon. (Buy One Here for under $80)