Portage Bar Above link would be your simplest bet for a Grumman. Not my add but will save you from drilling holes. Simply just bolts around the round thwart bar.
You could potentially always talk to one of the outfitter's before heading in to see if they have a spare one. I have not actually gotten one from them but I do believe Voyageur North has a couple they could lend. I had a conversation with them about it last year but never ended up needing to use it.