How to keep Pegasus Bamboo Roman Tub handle/fixture from twisting...

I am trying to install a Pegasus Bamboo Roman Tub Faucet set onto a fiberglass bathtub. I am using the pre-existing holes for the set.

As I am tightening the nuts underneath the fixtures, I can't seem to get it snug enough to prevent the spout or handles from twisting. Also, I don't want to over-tighten the nut at the risk of cracking the fiberglass. Currently, these things will twist out of alignment if I turn the handle with too much force. In the future, I know my wife or kids will try to tighten the handle too much, so I am trying to prevent the inevitable.

The Pegasus set comes with a foam like washer on the top side of the fixture (above the tub) and a rubber washer, brass washer, and brass nut on the bottom side (under the tub).

Help.

Normally, you just tighten up the lower nut and that would be good enough to hold it.
4.0 gallons a minute for a roman tub valve? That's going to take a while to fill.

Pegasus doesn't publish instruction sheets, so it's hard to know what you have.

Thanks, Terry. I haven't even considered the rate at which the tub will fill. The only (and I emphasize ONLY) factor I considered was the wife's selection.

At the moment, it seems like the foam like washer is not gripping to the top, smooth surface of the fiberglass tub. On the bottom side, the fixture is turning where the brass washer meets the brass nut. I will try to tighten the nut a little more.

As for Pegasus not publishing instruction sheets, my set might as well not have any. Instructions are horrible, but that's subjective on my part as a self-proclaimed DIY'er.

As a side note, the hoses included were short. Customer service ended up sending me longer hoses.

If we were dealing with a manufacture like Delta, Moen, Kohler, Grohe, Hans Grohe; you would easily find the parts breakdown and the instructions on their web sites.
A Roman Tub faucet shouldn't need much in instructions, however it would be nice if their shower valves had more complete support. The rubber washer should go up against the bottom of the tub in most cases, then the washer and then the nut.

the phone number for Pegasus support is 1-888-328-2383

How does the water inlet connect to the faucet? Usually it is a soldered connection which prevents ANY movement of the valve itself. The spout is a different situation if it connects with hoses, because its length provides extreme leverage, so most of those spouts are anchored with a positioning pin to prevent movement. IF there is no "top nut" and it depends on the escutcheon to prevent movement, it is NOT going to happen, especially after the first time you have to remove it to repair the faucet.