Well I also say no. I love my box and like I have said in other post till death do us part.
I recently put on new tires, the mich pilote sport A/S which are amazing. I go pretty fast in the rain with these and I took the box snow drifting with them as well. Simply amazing.
Back seats well don't care for them. Would be nice but its not a necessity.
The cup holder my friend bought it for me. Its one of the bell cup holders. Not great but its so much better then holding the drink or asking my friend to hold it for me.
Like the others said maybe all you need is a second car. I use my parants SUVs when I need to do something the box can't " carry more then one person, more stuff, and drive in snow higher then 2in."