A little pricey, but these worked great. I tried PVC and didn't find one that I thought fit well. One my ACE, for the cupholder on the drivers side and the two closest to the dash on the doghouse, you put the 3" side down, and it essentially forms a cupolder that 3" and flares out to 3.5".
Tall enough to hold bottles, and wide enough to hold large fast food drinks.
On the other two doghouse cup holders, these can also be used, but in reverse, where the wide part goes in cup holder, so the top would be narrower 3" than the bottom, but still good for bottles.
This is a radiator hose reducer, so you might be able to get it at an auto parts store, but I will say I ordered two brands in from Amazon, both 3.5 to 3" reducers and the other one was much more flimsy and didn't fit, where this one is perfect.