I recommend reinforcing the parts of the mast where you will be attaching hardware or sheaves and also where the mast meets the mast step and mast partner.

For my mast, I took a piece of closet rod and trimmed it to fit into the mast tube. I used a 20-inch piece in the bottom and a 10-inch piece in the top.

Remember, you do not have to be precise! Just get something in there that will hold the sides out. If you know the direction of the force, you could even just have a 1/4 inch thick slice of wood in there holding the opposing sides apart.