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Well, I don't have a good way to make and maintain multiple page formats for these – sorry – but I find that it's usually necessary to do a little trimming of the booklet pages anyway to get nice consistent margins (depending on your printer settings and capabilities), so I don't think it would save you much effort in practical terms. Most people need to trim a little anyway; you'd just trim off slightly different sized bits.

I recommend printing it as large as possible on A4 and then simply trimming off the ends to create consistent margins. If I have my math right, you'll trim 6.25cm off of each end, leaving a center of 27.2 cm or so, if you can print the US Letter source at the full width of A4.

Many printers can't print the full width of the page even when what's being printed has margins already, though, so you may need to adjust these to match whatever capabilities your printer has.

(I think this is why a lot of the fine folks who show the game on YouTube seem to have inconsistent margins on their copies – their printer couldn't print to the full size of the page and they didn't trim, so when they folded the booklet, it has the booklet margins plus the printable margins on the "outer" edges. I have yet to come up with a good solution on how to deal with that, even in US letter sizes, unfortunately.)

Anyway, folding the pages along the centers of the between-page margins will help clarify the size of those outer margins before you trim, if it's helpful. As long as each "page" is pretty close to being the same size, with the content in the center of its rectangle, it should fold up fairly nicely. (There will always be a little shift because of paper thickness when folded.)

In any case, thanks for your interest in the game, and I hope you enjoy it on your holiday!