I made one for a Thor with a round mouse hole. Rather, I got one of the shops at work (now I'm retired however) to punch it out on a computer controlled punch machine.
Not that hard to do by hand however, piece of aluminum about .050 thick or so , and sabre saw with a hacksaw blade and some files.
'03 Ram 2500 CTD, 5.9HO six speed std cab long bed Leer top and 2008 Bigfoot 25B21RB.. Previous, Being repaired and will be sold, is a 2008 Thor/Dutchman Freedom Spirit 180. SOLD - 2007 Winnebago View 23H Motorhome.