I need to build a small fence to surround a stack of round bales. It's in a corner of a field, so I need to fence in two sides.

I've bought some of those stand alone 20 ft corral panels that are out of the 2 7/8 inch pipe (I think that's the measurement). They seem pretty heavy.

For the end of the pen, I'm thinking of two of those panels that can be moved with a tractor to access the hay. But for the long side, I was wondering if I could put a panel on each end and stretch a short barbed wire fence between them with the fence tied to the panels on each end.
It would need to be maybe 100-150 ft long,

What do y'all think?

Would I need to secure the panels from pulling towards each other? Could the wind blow them out of place? Would a couple t-post hold them in place ok?

I'm thinking that would be a cheaper approach than buying any type of panels. And I'm not sure that would be a permanent fence, so I don't want a real corner post set in the ground.

Electric fence works great .
I used it for fencing off about 100 round bales each year - for over a dozen years until I sold out.

Simple and easy with solar charger and two strands of string type wire ( Gallagher is the best), and step in plastic posts.
I used T-posts at the "corners".
I set up a " gate" with those insulated spring loaded handle at each end so I could let one side dry up in wet years.

Also used electric fencing for strip grazing when doing pasture renovation plantings and also forcing cows to graze shaded grass in my pecan orchard.

I think your plan would work fine but yes, I would anchor them somehow. T-posts should work fine. The rest depends on just how hungry the cows get. If they are motivated then they will find a way.

2 7/8" tubing (oilfield pipe) is very heavy (thick walled). Too heavy for this application without bracing. Perhaps you have galvanized tubing? If so, it should work fine with t-posts support.