Disclaimer - amateur woodworker and installer at work.
My '21 XL Boondock did not come with a sliding tray and I wanted one to go with the new Alpicool C30 cooler I had purchased.
I started by purchasing a set of heavy duty slide rails from Amazon:
and then I purchased a 3/4" board from Lowes with I cut to fit the space at 17.5" wide and 25" long. I spray and stained the board with the closest color I could find. I also added a small lip to the board (which turned out to be a saver when it comes to opening the tray).
Once I got going I quickly realized that the installation was going to be tight as I measured the space - there is a drop towards that back of the storage space which limits a cooler height. I had 2 1/2" of clearance to work with.
When I went to install the setup I ran into a couple of issues - there is a 1 1/2" of incline from the back of the space to the back of the trailer, narrowing my 2 1/2" of usable space. There is also a raised bracket for locking the back door that you have to clear. With a 3/4" board, I knew it was going to be tight. Also, you will need to bring the board about 7" out from the end of the rails to have enough clearance to open the cooler when fully deployed.
When I went to install the setup I also found I needed to do it in stages - the left rail was secured using two existing holes and one new one, then I had to attach the right rail to the board before inserting the whole unit to the space. I then attached the left rail slide to the board, leveled the board and then attached the right rail to the unit.
Turns out I was about 2/8th of an inch off when I tried to slide the cooler fully into the enclosure. I was able to fix this by removing the back rubber feet of the cooler and now I can use the space as intended.
I'll probably get a 1/2" board and redo the base with it so I can put the rubber feet back on the backside of the cooler so I dont have that nagging feeling its not fully complete.
Total time was a few hours for me, so if I can do it I have confidence that you can as well.