I FINALLY ADDED AN ARMREST
This mod has been long overdue and I'm so happy to have finally just done it. I didn't take photos of the installation because I wasn't sure how it'd turn out, but I really shouldn't have been worried.
I started by measuring the height of the armrest on the door in relation to the pivot point on the seatback, then marked this location on the fabric on the other side of the seat with a chalk pen. This gave me a top-of-armrest reference. Next, I removed the seat cover and foam (if you have fabric seat covers take this opportunity to toss them in the washing machine) and marked the location of the top of armrest on the seat frame. Then I mocked up the armrest on the post and marked where the plate would need to be mounted on the seat frame.
After that, I clamped the post bracket in place, replaced the foam to test fit its orientation, and made adjustments in the X & Y axis until I was happy. I had to grind and bend the post mounting plate to conform to the seat frame somewhat. Once that was done I marked and drilled one hole through the seat tube to bolt the plate in the correct orientation. After the final test fit, I burned it in with the MIG.
Cutting a rough hole in the fabric is a bad idea as it will eventually fray, I thought about sewing something along the circular cut to prevent fraying but ultimately went with an easier and more elegant solution.
I started by putting the fabric cover on the foam over the frame and stretched it to mimic the final position it will be in when tight, then I marked out the post location on the inside of the fabric and used a pick to punch a hole through the center of that. Next, I clamped the fabric taught over a block of wood in front of my exhaust vent fan. I grabbed a throw-away 3/4" deep 3/8" drive socket put it on an extension, heated it up with my torch (DONT BREATH THE FUMES IN) then I centered the socket over the post hole and burned the hole in through the fabric. I kind of wish I had taken some photos because this method was very easy, effective, clean, and will prevent fraying.
So sorry about the lack of photos but here's the end product. This is the armrest I used:
Central Armrest Seat Arm Rest