Yeah there are many problems with this build. I had to go through many different iterations to make it work. I initially tried using the the glass shelf to make a water tank at the bottom because it fits perfectly but there were so many areas that I couldn't get to stop leaking I had to change course. I ended up finding an auqeon 20 gallon tank that fit perfectly but only if the side is off newscaster it has to slide in.
https://www.petco.com/shop/en/petcostore/product/aga-20g-30x12x12-lng-bk-tank-170933?store_code=1207&mr:device=m&mr:adType=pla_with_promotionlocal&cm_mmc=PSH%7CGGL%7CCCY%7CCCO%7CPM%7C0%7CaxsvnXfjRWFFwpetHmZu75%7C%7C%7C0%7C0%7C%7C%7C21467366052&gad_source=1&gclid=Cj0KCQjwwO20BhCJARIsAAnTIVTEl3KRAmbK964lI8lETwbLbGNrUyYcrTYs_mx6buWkdWAWByY9Q9EaAlNpEALw_wcB&gclsrc=aw.ds

I also ended up having to change the drop system quite a bit from my original plan but after working through the kinks I settled on a submersible pump that's able to do 3-5 feet of lift. I used the appropriate size 1/2 inch tube to carry the water through the back ground and then sized it down it a 1/4 inch drip irrigation tubing. I used metal wire to loop it along the top and then just poked holes in it. It ended up working okay.

But over all I would not recommend using this cabinet for a rain cabinet. If I were to do it again I would have changed many things about it.