I haven't seen anyone make a custom variant in quite a while. Best option is going to be ordering this:
https://www.ebay.com/itm/2013-BRZ-FR...Condition=3000
Pull it apart to get the boot then pull the stitching on the boot to get your piece patterns.
Would actually make for a nicer gift, as you could reassemble it once you're done, so all he has to do is pull the old trim piece and clip the new one in.
I figure you'd only need about 1 yard of fabric to make a boot, so as long as it's nothing exotic, should be able to do the entire project for less than $70 all-in, which is really good for anything custom for cars.