I wouldn't worry about saving a few grams on a 1980s steel bike. It's not like Phil Gaimon trying to save grams on a 12 lb climbing bike. I barely notice the weight difference on my '89 Centurion Ironman with or without a second full water bottle, even on hill climbs.

And don't depend on zip ties for long. Especially not to hold a water bottle. I've seen those snap and nearly cause crashes in group rides when other riders used zip ties to secure their water bottle cages.

I use them to secure the brake cables on my road bike -- the calipers are very picky about cable position. The zip ties secure the cables so the pad doesn't rub on one side. But riding about 400-500 miles a month mostly in daylight the sun breaks down the zip ties pretty quickly. They'll get brittle and snap every 2-3 months. No big deal, doesn't cause any serious problem, just minor brake pad rub on the rims.

I carry spares in the seat bag. Same age as the zip ties on the handlebar and frame, but still pliable because they aren't exposed to sunlight.

A hose clamp would be a much safer and more dependable long term solution if nothing else is readily available.