I have successfully held the back end of many larger tanks in securely with large industrial velcro, and battery packs as well. If you have room for two servings, you'll be good to go. You may want to look inside of any large IMAC plane, you would be amazed at how many of us are using velco to hold in 20 to 32 oz. tanks. My test is, if you can pick the fuselage up by the tank, then it is most likely strong enough.

That being said, do not us light weight velcro, with only one serving, you may regret it. If you can't pull it out with the velcro attached, then you are most likely okay. For smaller planes like yours, I usually opt for a light ply brace, with foam glued onto it. I make a notch support glued to fuse, and fasten it with a couple of small (servo) screws for easy removal in the field. As sure as you glue it in, you will drop a clunk, have a fuel line leak or need to pull the tank for some reason, and have problems doing so.

Good luck with your LT40, it is a great, rugged, reliable trainer.