Making own grip balls for pullups

Hey just looking for some advice.
I am making some grip balls (like the fbbc bomb) with tennis balls, eye bolts and a bit of rubber tire glue.
I cut a slit into the ball, squeezed through the nut to the eye bolt with a wide circle of metal, then screwed the bolt in and glued up the slit.
I have only made one so far as didn't want to ruin two balls if the idea didn't work. I haven't tested out its strength yet but know that without glue the ball cant even hold 15kg's.
Thinking along the lines of stitching the ball to hall the bolt in tight, but that will be very difficult as the rubber is thick.
Also, does anyone know if a big staple gun would hold the ball together?

many thanks, appreciate any ideas

Thanks egg,
yep they do compress but aslong as they can hold my weight that doesn't really matter to me. I did think abt baseballs, however i only had tennis balls at hand.
Where did you get the baseballs?
Did you need to cut the baseball at all, if not how did you screw in the eyebolt (forgive me i am new to certain DIY things but really enjoy making these stuff).

Might also see abt attaching them to my home made sled and see how that feels.

I made a cheap one using aplastic baseball from a toy set (from Asda). Drilled a hole and put a big eye bolt through it with washers on either side. The plastic makes it especially challenging because of the smoothness.

thanks for the replies.
Don't the washers on the other side of the ball make it a bit difficult for griping?

Used some 2.5" pvc pipes for pullups today and some towel pullups, it was really great, much more interesting than regular pullups, plus better wrist position can be made.