I don't permanently carry much extra on my tractor, only a roll of TP. Otherwise, I throw the tools I'm going to need for a days work in the bucket. There are several threads where extra carry space was made in the 3-point hitch weight box.
There is a thread with people carrying chains etc in PVC pipe attached to the loader bracket.
There are threads if larger tool boxes between the cab safety frame members (ROPS)
There are threads of people making rear balance weights out of barrels with 'sockets' or PVC pipes down as tool carriers and the top 10" left open for more tools.
I'm sure more ideas exist but these come to mind quickly, have a search and a read.