You should be able to get your mate to build one out of say 1 1/2” box section, i'd ensure its 2mm wall or larger. If you have a look at a standard ROPS on other machines, it should give you a guide. Hopefully you may even have mounting points where the one that hopefully it did have at one point was mounted. They aren't too complicated, i would just ensure all the corners had gussets just in case it did roll, you don't want it to simply bend at the joins and fold out offering you little to no protection... You can use punched mesh on the top and sides, but i assume its mainly just incase you roll over? if its just for roll over, you technically don't need mesh on the sides. On the roof, i honestly would, you don't want a rock or clump of dirt hitting you on the nogg'in....
Hopefully another member in here has one with a cage that can take a picture so you can get a better idea of how they are fitted.