Looking for a source for a small basic slide in. Would even take a used one as I have to make extensive modifications for work.

Looking for something that is literally one step up from a topper, an empty but weather tight shell. doesn't have to go over the cab as it will be on a regular cab with a 6.5' bed. Don't have the truck yet, but looking for something basic in the 2015 or newer range, preferably 2018 or newer if the mileage isn't too high and the seller doesn't think it's gold plated. Will be putting tons of miles on it as I'm on the road over 300 days a year. Saw some decent prices for some XL Fleet units on line.

Any direction or hints you could pass on would be greatly appreciated.