I dont have said extra mounts so purchased some hook and look from 3M... This specific stuff isn't velcro on one side and the fluffy stiff on the other but is like a mushroom effect and once hooked into place (on itself) is very very strong. I actually tried pulling the bike up with the stembag insitu and it held (more on that later).
So... Bought this - https://www.amazon.co.uk/3M-Scotch-3550 ... B00D6OI99G - but from fleabay so cheaper but maybe fake! Stuck one side to the frame top tube and other to my waterproof welded stembag (think Apidura but this ones by Raph% themselves and is impressively waterproof) underneath. Used a heatgun on both applications, bike and frame....
Bit busy in above pic so heres another shpwing the actual concept and how it went on my 109 mile(very rainy BAM for this month) expedition, then another 48 mile groupride.
As you can see, no strap fastenings needed on both the stem nor the toptube...
Below is how it (
) look when removedshould
Ok... So did it work.
andyes
. It managed the 109 miles -in the pouring rain and some wind on the road bike -before I chucked in the towel cos I had the opportunity of a lift. Thats when the flaw showed its ugly teeth. When I tried to remove the bag so we could strap bike to his boot,kindof
.the actual hook/loop proved stronger than the sticky tape backing* and when peeling yhe bag off carefully, the part stuck to bag stayed on frame
However, after that cos there were still remnants still on the bag, I managed to salvage it and ride another 48 miles a few hours later
... Hence, with a bit of thought on how to get a secure bond to the bag, I should have me a nice strapless bag 
*VHB apprently which stands for very high bond, hence why I queried whether its a fake cos the original VHB tape by 3M is just that.
... I thought they were the highlight buttons for some reason...