I know you want it light, and there are many great light tripods. I would suggest a fluid head. If you spend hours glassing, the fluid head is worth the extra weight. They are so much smoother and it is much easier on your eyes and easier to correctly cover an area while glassing. Please give as much thought to the tripod head as you do the tripod itself.
Fluid heads are definitely smoother to use.
But some of the regular heads are much smoother than others, even when they are the same exact head.
And they can be a bit jerky when compared to a fluid head.
But does this mean they arent usable?
No it dosent, and for side to side movement, (panning) you dont have to use the head at all, just loosen the tripod stem a bit.
I have over the many years ive been using tripods for glassing, bought lots of them on Ebay.
The Monfrotto brand offers lots of different models.
Even the smaller ones easily support our twin spotter binoculars, and can be bought used very reasonably.
They can also be found without any head, allowing you to buy that separately.
$100 will probably get you a very good tripod on Ebay.
As an example, a couple years back i was browsing there and found a monfrotto walking stick. Which is a 3 section heavy duty leg of a tripod, with a rubber grip and a stud for connecting a spotting scope, or even a device for supporting a rifle. The starting bid was 10 dollars, no body bid on it, except me, and i bought it for 10 dollars in new condition.