70-300mm in vertical orientation for under $50. That is a tough order. At that price point your best option would be a consumer video tripod with center column brace. Slik, Sunpack, and various other brands offer products in this space. I used one of those units (Slik U-8000 @ $45) for several years before the head became too worn to hold without slipping.
Tripods of that sort are adequate and will do the job. The main issue is flexing of the legs when the load is off-center (i.e. in vertical orientation). There is also the matter of resistance to vibration. Addition of weight, either suspended or wrapped around the crown helps with vibration, but not the flexing. Other issues include:
- Bulky
- Limited options for low angle configuration
- Very poor quality head
- Very breakable plastic hardware
I am sorry to say that despite my being one of the worlds foremost cheapskates, I have not been able to make the "quality at reasonable weight" price point at less than about $100 with the true bottom point being closer to $120.
BTW...the issue with your zoom is not so much weight as it is focal length. Even small vibrations (say from wind or mirror slap) at 300mm are amplified due to the lens' reach.
Steve