I'll try to get some photos when I get home but here is a general description.
Topeak Trunkbag Ex-Velcro (
https://www.amazon.com/gp/product/B0186OQIEI/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1)
The MIK adapter plate (
https://www.amazon.com/Basil-MIK-Adapter-Plate-Black/dp/B075DKV1MK)
View attachment 36373 View attachment 36374
1.Each corner is a screw & washer that screws into the base. Red circle is the piece that screws loose.
2. I took the screws out so I could mark the locations of the holes on the bottom of my Topeak bag. I taped the base in place and snapped it in to make sure I had it where I wanted it. The bottom is semi ridgid so that really helps to mark & then drill through it.
3. I took the bottom padding & taped it out of the way inside the Topeak bag (just a small piece that flips up. and drilled my marked holes (going from the outside to the inside)
4. Then place the screws/Washer pieces on the inside the bag, through the holes you just made, and attached to the base of the MIK plate. Tighten screws & you are done!
I was super nervous drilling holes in a brand new bag... but it worked out well.
Earlier i had posted the wrong bag, mine doesn't have the rail system on it. I got the velcro one, and the velcro is not in the way at all. I edited the posts to show the correct Topeak Trunk bag.